Set a default for add-on startup config

[frenck]

Proposed change

Set a default for the add-on startup configuration, making the configuration parameter optional.

The default is set to application. Looking at the add-ons, most use service, however, I think application is a sane and safe default. Add-ons that have a more specific startup requirement can still use this option of course.

This PR allows cleaning up add-on configurations a bit more and is backward compatible.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to the supervisor)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: Set a default for add-on startup config developers.home-assistant#770

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast supervisor tests)
• Tests have been added to verify that the new code works.

If API endpoints of add-on configuration are added/changed:

• Documentation added/updated for developers.home-assistant.io