qos: T6638: require interface state existence in verify conditional (backport #3960)

[mergify]

Change Summary

Add an optional strict requirement of interface state existence to verify_interface_exists: this has no effect unless keyword state_required=True, as needed in qos.py in case of a late arriving pppoe interface.

This restores the intended behavior of the conditional in qos.py:
ed63c9d#diff-c3e850625edc3ba2f44020fcda8640d00631b3c67bac756f258b919667b9d67eR306

The obvious question of how to approach re-applying qos.TrafficShaper.update when the interface is available (other than providing a warning to the user) is a separate issue which will need to be addressed in a subtask, if not obvious to those more familiar with the subsystem.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Related PR(s)

Component(s) name

Proposed changes

How to test

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

---

This is an automatic backport of pull request #3960 done by [Mergify](https://mergify.com).

[github-actions]

👍
No issues in PR Title / Commit Title

[github-actions]

👍
No issues in unused-imports