Well, further investigation showed to me that maas boots with:
   ip=off ip6=dhcp
   ip=dhcp ip6=off
but will never boot with either
   a.) ip=dhcp ip6=dhcp
   b.) ip6=dhcp

An oddity in the initramfs implementation is that 'ip6=dhcp' will enable
ip=dhcp.  Which seems arcane, but since ip=off is available, it does at
least allow the user to imply sane behavior.

Because maas will never do both ipv6 and ipv4, then in their code path
the initramfs will never creates a /run/net-eth0.conf and
/run/net6-eth0.conf in their environment.  That was the case that the
changes here fixed.  So, my statement in comment 6 was incorrect.

However, I have verified that this actually does work via some manual
testing using qemu and some work i'd done for open-isci testing.

We boot a qemu system like this:
qemu-system-x86_64 -enable-kvm -device virtio-scsi-pci,id=virtio-scsi-xkvm 
-device virtio-net-pci,netdev=net00 -netdev 
type=tap,id=net00,script=no,downscript=no,ifname=tapvm0 -echr 0x5 -m 1024 
-nographic -kernel /tmp/tgt-boot-test.UT2p0T/kernel -initrd 
/tmp/tgt-boot-test.UT2p0T/initrd -append "nomodeset iscsi_initiator=maas-enlist 
iscsi_target_name=tgt-boot-test-UT2p0T 
iscsi_target_ip=10.5.219.1iscsi_target_port=3260 iscsi_initiator=maas-enlist 
ip6=dhcp ro net.ifnames=0 BOOTIF_DEFAULT=eth0 
root=/dev/disk/by-path/ip-10.5.219.1:3260-iscsi-tgt-boot-test-UT2p0T-lun-1-part1
 overlayroot=tmpfs console=ttyS0 
ds=nocloud-net;seedfrom=http://10.5.219.1:32600/";


** Attachment added: "verification log on xenial"
   
https://bugs.launchpad.net/cloud-init/+bug/1639930/+attachment/4792426/+files/verify.txt

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Title:
  initramfs network configuration ignored if only ip6= on kernel command
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