lp:~tai271828/curtin

Author: Scott Moser
Author Date: 2018-11-02 22:30:22 UTC

apt: Use new format apt config when writing preserve_sources_list.

cloud-init reads the "preserve_sources_list" setting from 2 config paths:
 old: top level apt_preserve_sources_list
 new: apt/preserve_sources_list

Previously curtin would write only the old form. That lead to the
case where cloud-init would raise exception complaining about conflicting
settings and exiting error. That occurred because MAAS was incorrectly
sending 'preserve_sources_list: True'

The change here could result in actually causing that problem if:
A. a user was specifying old style 'apt_preserve_sources_list=False' in
   user-data. They now have to match the version that curtin writes
   but at least curtin is writing new format on new cloud-init.
B. MAAS provided differing setting in the curtin config
  'debconf_selections'

That is because now the system will have format config specifying True
written by curtin and a different format written by user/maas.

I'd suggest the most likely chance for regression here would be
16.04+ systems with old MAAS (providing 'B') or JuJu providing 'A'.

LP: #1735950