I was also able to reproduce this, here are my notes for now:

## /etc/fstab

LABEL=LVROOT / ext4 errors=remount-ro 0 1
LABEL=LVVAR /var ext4 defaults 0 1
LABEL=TESTE /teste ext4 defaults 0 1

Right after boot:

inaddy@trustylivepatch:~$ systemctl list-units --all | grep fsck
systemd-...el-LVVAR.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-LVVAR.service
systemd-...el-TESTE.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service

This indicates that UPSTART is the one mounting the block devices, NOT
SYSTEMD using its mount units. SNAPS are mounting the SQUASH filesystems
using SYSTEMD UNITS, despite UPSTART scripts.

It is likely that this wasn't noticed, on systems mounting "/" only,
because the "-.mount" SYSTEMD UNIT doesn't depend on "systemd-
fsck@.service" unit, it depends only on "systemd-fsck-root.service", non
existent in TRUSTY's SYSTEMD version. Probably this made SYSTEMD to act
like no error existed.

For SYSTEMD mount units to work, it is needed that no fsck unit error
exists - like when having /var or any other mounting besides root
filesystem - allowing all SYSTEMD units created by snappy to work.

Comparing default setups for TRUSTY and ZESTY:

--------
## TRUSTY

$ dpkg -L systemd | grep fsck
/lib/systemd/systemd-fsck

$ systemctl list-units --all | grep fsck
systemd-...el-LVVAR.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-LVVAR.service
systemd-...el-TESTE.service error  inactive dead    
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service

$ systemctl list-units --all | grep mount
-.mount            loaded active   mounted /
teste.mount        loaded active   mounted /teste
var.mount          loaded active   mounted /var
umount.target      loaded inactive dead    Unmount All Filesystems

## ZESTY

$ dpkg -L systemd | grep fsck
/lib/systemd/system/systemd-fsck-root.service
/lib/systemd/system/systemd-fsck@.service
/lib/systemd/system/systemd-fsckd.service
/lib/systemd/system/systemd-fsckd.socket
/lib/systemd/systemd-fsck
/lib/systemd/systemd-fsckd

$ systemctl list-unit-files | grep fsck
systemd-fsck-root.service                       static
systemd-fsck@.service                           static
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service static
systemd-fsckd.service                           static
systemd-fsckd.socket                            static

$ systemctl list-unit-files | grep mount
-.mount                               generated
home-inaddy-work.mount                generated
mnt.mount                             static
mountall.service                      masked
umountfs.service                      masked
umountroot.service                    masked
umount.target                         static
--------

SYSTEMD in TRUSTY was treated differently for FSCK. TRUSTY's version
contains systemd-fsck but not systemd-fsckd, the daemon responsible for
consolidating all fsck information for SYSTEMD journal. It is also clear
that TRUSTY did not include any unit file for systemd-fsck@.service,
that might still be considered for the automatically generated mount
unit files.

You can reproduce this by trying to use TRUSTY SYSTEMD mount units:

--------
## TRUSTY

$ mount /teste

$ mount | grep teste
/dev/sdb2 on /teste type ext4 (rw)

$ systemctl status teste.mount
teste.mount - /teste
   Loaded: loaded (/etc/fstab)
   Active: active (mounted) since Wed 2017-09-27 17:27:21 BRT; 10s ago
    Where: /teste
     What: /dev/sdb2
  Process: 1754 ExecUnmount=/bin/umount /teste (code=exited, status=0/SUCCESS)

$ systemctl stop teste.mount

$ systemctl status teste.mount
teste.mount - /teste
   Loaded: loaded (/etc/fstab)
   Active: inactive (dead) since Wed 2017-09-27 17:27:33 BRT; 2s ago
    Where: /teste
     What: /dev/disk/by-label/TESTE
  Process: 1778 ExecUnmount=/bin/umount /teste (code=exited, status=0/SUCCESS)

$ mount | grep teste

$ systemctl start teste.mount
Failed to issue method call: Unit 
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service failed to load: No such file or 
directory. See system logs and 'systemctl status 
systemd-fsck@dev-disk-by\x2dlabel-TESTE.service' for details.
--------

Also, in TRUSTY, SYSTEMD creates the mount units based on /etc/fstab
entries, just like the recent SYSTEMD does, but on them THERE IS NO
setting for fsck dependencies (Requires/After), and, still, it appears
to be considering those fsck dependencies when you try to mount a
".mount" unit, based on errors I showed above for "systemctl start
XXX.mount".

--------
## TRUSTY

$ cat var.mount

# Automatically generated by systemd-fstab-generator

[Unit]
SourcePath=/etc/fstab
DefaultDependencies=no
After=local-fs-pre.target
Conflicts=umount.target
Before=umount.target
Before=local-fs.target

[Mount]
What=/dev/disk/by-label/LVVAR
Where=/var
Type=ext4
FsckPassNo=1

## ZESTY

$ systemctl edit --full mnt.mount
[Unit]
SourcePath=/etc/fstab
Documentation=man:fstab(5) man:systemd-fstab-generator(8)
Before=local-fs.target
Requires=systemd-fsck@dev-disk-by\x2dlabel-TESTE.service
After=systemd-fsck@dev-disk-by\x2dlabel-TESTE.service

[Mount]
What=/dev/disk/by-label/TESTE
Where=/mnt
Type=ext4
--------

Unfortunately creating the fsck service unit didn't seem to help for
TRUSTY, and, according to this:

$ for file in `dpkg -L systemd`; do [ -f $file ] && grep systemd-fsck $file; 
done
Binary file /lib/systemd/systemd matches
Binary file /bin/systemd matches

It is extremely likely that systemd-fsck@.service was hardcoded as a
dependency for systemd mount unit files, and the fsck units systemd-
fsck@<DISK>.service are NOT BEING CREATED automatically. This is the
real problem to be fixed IMO.

This case should be targeted to both: snapd (trusty) AND systemd
(trusty)

** Changed in: snapd
     Assignee: (unassigned) => Rafael David Tinoco (inaddy)

** Changed in: snapd
       Status: New => Confirmed

** Also affects: systemd (Ubuntu)
   Importance: Undecided
       Status: New

** Changed in: systemd (Ubuntu)
     Assignee: (unassigned) => Rafael David Tinoco (inaddy)

** Changed in: systemd (Ubuntu)
   Importance: Undecided => Medium

** Changed in: systemd (Ubuntu)
       Status: New => In Progress

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https://bugs.launchpad.net/bugs/1718966

Title:
  Cannot install snaps on Ubuntu 14.04 with /var on its own partition

Status in snapd:
  Confirmed
Status in systemd package in Ubuntu:
  In Progress
Status in systemd source package in Trusty:
  New

Bug description:
  Installing snaps is not possible on Ubuntu 14.04 when having /var on
  its own partition, whether its LVM or not.

  The issue is with the core snap being unable to mount:

  The error with /var isolated and using LVM:

  root@ubuntu:~# snap install canonical-livepatch
  error: cannot perform the following tasks:
  - Mount snap "core" (2898) ([start snap-core-2898.mount] failed with exit 
status 6: Failed to issue method call: Unit 
systemd-fsck@dev-mapper-vg00\x2dvarvol.service failed to load: No such file or 
directory. See system logs and 'systemctl status 
systemd-fsck@dev-mapper-vg00\x2dvarvol.service' for details.
  )

  The error with /var isolated but without using LVM:

  root@ubuntu:~# snap install canonical-livepatch
  error: cannot perform the following tasks:
  - Mount snap "core" (2898) ([start snap-core-2898.mount] failed with exit 
status 6: Failed to issue method call: Unit 
systemd-fsck@dev-disk-by\x2duuid-7383abd2\x2d019c\x2d46c2\x2d8b36\x2d34633cc8f3ca.service
 failed to load: No such file or directory. See system logs and 'systemctl 
status 
systemd-fsck@dev-disk-by\x2duuid-7383abd2\x2d019c\x2d46c2\x2d8b36\x2d34633cc8f3ca.service'
 for details.
  )

  The same error happens if I try to install the hello-world snap (with
  LVM in this example):

  root@ubuntu:~# snap install hello-world
  error: cannot perform the following tasks:
  - Mount snap "core" (2898) ([start snap-core-2898.mount] failed with exit 
status 6: Failed to issue method call: Unit 
systemd-fsck@dev-mapper-vg00\x2dvarvol.service failed to load: No such file or 
directory. See system logs and 'systemctl status 
systemd-fsck@dev-mapper-vg00\x2dvarvol.service' for details.
  )

  I cannot reproduce the issue in Ubuntu 16.04.

  I couldn't reproduce this issue by using the Ubuntu 14.04 cloud image
  which doesn't isolate /var on its own partition. I tried adding a
  secondary disk to that cloud image VM and create a dummy VG and LV,
  but couldn't reproduce the issue.

  Also could not reproduce using Ubuntu 14.04 (with LVM or not) but with
  only a / partition and swap.

  
  Steps to reproduce:
  ===================

  # Install Ubuntu 14.04 in KVM (I used the 14.04.4 server iso) and
  configure /, /var and swap on their own partitions (with LVM or not,
  the issue happens in both situations).

  root@ubuntu:~# lvs
  LV VG Attr LSize Pool Origin Data% Move Log Copy% Convert
  rootvol vg00 -wi-ao--- 3.72g 
  swap vg00 -wi-ao--- 3.72g 
  varvol vg00 -wi-ao--- 3.72g 

  root@ubuntu:~# df -h
  Filesystem               Size  Used Avail Use% Mounted on
  udev                     484M  4.0K  484M   1% /dev
  tmpfs                    100M  988K   99M   1% /run
  /dev/dm-0                3.7G  1.7G  1.8G  49% /
  none                     4.0K     0  4.0K   0% /sys/fs/cgroup
  none                     5.0M     0  5.0M   0% /run/lock
  none                     497M     0  497M   0% /run/shm
  none                     100M     0  100M   0% /run/user
  /dev/mapper/vg00-varvol  3.7G  716M  2.8G  21% /var

  
  # Upgrade system, install snapd and reboot

  root@ubuntu:~# apt update
  root@ubuntu:~# apt upgrade -y
  root@ubuntu:~# apt install -y snapd
  root@ubuntu:~# reboot

  
  # After reboot, check kernel version and try to install the 
canonical-livepatch snap:

  root@ubuntu:~# uname -a
  Linux ubuntu 4.4.0-96-generic #119~14.04.1-Ubuntu SMP Wed Sep 13 08:40:48 UTC 
2017 x86_64 x86_64 x86_64 GNU/Linux

  root@ubuntu:~# snap list
  No snaps are installed yet. Try "snap install hello-world".

  root@ubuntu:~# snap install canonical-livepatch
  error: cannot perform the following tasks:
  - Mount snap "core" (2898) ([start snap-core-2898.mount] failed with exit 
status 6: Failed to issue method call: Unit 
systemd-fsck@dev-mapper-vg00\x2dvarvol.service failed to load: No such file or 
directory. See system logs and 'systemctl status 
systemd-fsck@dev-mapper-vg00\x2dvarvol.service' for details.
  )

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