feat: add autopkgtests assert no warnings from cloud-init status

[blackboxsw]

Introduce autopkgtests for cloud-init to validate runtime of cloud-init status.

Proposed Commit Message

feat: add autopkgtests assert no warnings from cloud-init status

Additional Context

Note this branch will currently "fail" with warnings for two known reasons:

1. Awaiting fix in debian/cloud-init-base.postinst and .config fixes[1] to avoid cloud-init-base.postinst creating invalid YAML in /etc/cloud/cloud.cfg.d/90_dpkg_local_cloud_config.cfg
2. Awaiting upstream fix[2] for autopkgtest to stop creating deprecated apt_preserve_sources_list keys in autopkgtest systems during /usr/share/autopkgtest/setup-commands/setup-testbed script.

The second issue can be avoided by changing the string in setup-testbed before running autopkgtest-build-lxd ...

'apt_preserve_sources_list: true'

TO

'apt: { preserve_sources_list: true }'

[1] cloud-init postinst/config fix: #6598
[2] autopkgtest upstream fix https://salsa.debian.org/ci-team/autopkgtest/-/merge_requests/621

Test Steps

# Make sure your test environment has autopkgtest and lxd installed
gh co 6601

# maybe manipulate your host system's /usr/share/autopkgtest/setup-commands/setup-testbed avoiding apt_preserve_sources_list key

autopkgtest-build-lxd ubuntu-daily:questing
autopkgtest --shell --apt-upgrade /home/csmith/src/cloud-init -- lxd autopkgtest/ubuntu/questing/amd64

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[blackboxsw]

cc @dbungert for visibility in case of autopkgtest input/suggestions.