tests: pro integration tests supply ubuntu_advantage until pro v32

[blackboxsw]

Until ubuntu-advantage-tools version 32 is SRU'd to stable series Pro images will continue to auto-attach by default unless an ubuntu_advantage key is provided in user-data.

Allow our integration test to provide ubuntu_advantage and ubuntu_pro keys knowing that any ubuntu_pro config overrides ubuntu_advantage in latest cloud-init.

This will emit a warning message about preference of ubuntu_pro over ubuntu_advantage and deprecation warnings, but the integration test doesn't validate deprecation warnings.

We can drop ubuntu_advantage from our user-data in May when Pro 32 releases.

Proposed Commit Message

<type>(optional scope): <summary>  # no more than 72 characters

A description of what the change being made is and why it is being
made if the summary line is insufficient.  This should be wrapped at
72 characters.

If you need to write multiple paragraphs, feel free.

Fixes GH-NNNNN (GitHub Issue number. Remove line if irrelevant)
LP: #NNNNNN (Launchpad bug number. Remove line if irrelevant)

Additional Context

Test Steps

Checklist

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

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>)

[TheRealFalcon]

LGTM!