netplan status --diff fixes and improvements

[daniloegea]

WIP

Description

Checklist

• [ ] Runs make check successfully.
• [ ] Retains 100% code coverage (make check-coverage).
• [ ] New/changed keys in YAML format are documented.
• [ ] (Optional) Adds example YAML for new feature.
• [ ] (Optional) Closes an open bug in Launchpad.