Wait for kine to be ready during startup

[twz123]

Description

So k0s will not continue with its startup sequence, and will exit with an error message indicating that kine was not okay.

See:

• k0s doesn't fail gracefully on broken kine config #4693
• dial unix /run/k0s/kine/kine.sock:2379: connect: no such file or directory #5079

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[twz123]

This is of course superfluous, as this is what the manager.Ready interface and the Ready method are for in the first place 🙄