Allow for static port in Watchdog add-on configuration

[frenck]

Proposed change

This PR add support for a static port configuration to the Watchdog URL add-on configuration.
Currently, it requires a defined port (while this should not be needed in most cases).

Fixes: hassio-addons/addon-zwave-js-ui#49

21-02-19 14:01:41 WARNING (MainThread) [supervisor.store.data] Can't read /data/addons/git/40817795/zwavejs2mqtt/config.json: does not match regular expression for dictionary value @ data['watchdog']. Got 'http://[HOST]:8099/health/zwave'

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:

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