current base cloud-image does not have resolvconf, but has ifupdown.

The following happens:

1) cloud-init starts
2) picks ifupdown renderer
3) renders eni
4) which uses auto dhcp
5) which calls dhclient
6) because there is no resolvconf installed, default make_resolve_conf function 
is used which is internal to isc-dhcp
7) /etc/resolv.conf stops being a symlink and is a regular file
8) NM starts, notices that /etc/resolv.conf is a regular file and thus is not 
managed by resolvconf/resolved
9) NM decides to manage /etc/resolv.conf, poorly

This does kind of mean that on upgrades, if one removes resolvconf without 
removing ifupdown things go pear shaped.
One option is to re-introduce ifupdown dependency on resolvconf.

Another option is to ship isc-dhclient (dhcp-enter-hook) in systemd to
override make_resolve_conf function. I've started writing such a hook
which uses busctl to feed per-link dhcp info to resolved. However, that
has it's own problems. dbus is not started early enough, and there is no
private socket to resolved, meaning that updates are getting lost on
boot when [email protected] units are udev triggered on hotplug. Currently
working on preserving state in /run and replaying isc-dhclient gathered
settings to resolved.

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https://bugs.launchpad.net/bugs/1716833

Title:
  NM manages /etc/resolv.conf directly in artful after removal of
  resolvconf

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