test: assert that schema is valid by default

[holmanb]

Proposed Commit Message

test: assert that schema is valid by default

Additional Context

This should increase our test coverage of cloud-init schema by running it by default in verify_clean_boot(). This command has had numerous tracebacks over the years (I just reported another), so additional coverage should hopefully help prevent this type of thing from getting hit by users.

My plan is to fix any new test failures caused by this new addition prior to adding verify_clean_boot() next to our verify_clean_log() calls - which should pave the way for eventually replacing verify_clean_log().

Note: In a followup PR I plan to add new flags (ignore_deprecations and require_deprecations) to allow skipping deprecation on tests which we intend to continue using deprecated keys and requiring a deprecation to be reported in the case where it should be.

Test Steps

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>)

[aciba90]

LGTM, thanks!