Restore correct auth source precedence for kubelet bootstrapping

[twz123]

Description

When introducing the k0s based kubelet kubeconfig bootstrapping, k0s defaulted to honor the join token argument over already existing kubeconfig files. This gives problems in conjunction with k0s install, as it bakes the join token arguments into the k0s service definition. So each time k0s gets started as a service, the join token argument will be present and tried to be used, even if the bootstrapping took place already.

Fix that by restoring the old precedence:

1. Use an existing kubelet kubeconfig file if present.
2. Use an existing kubelet bootstrap kubeconfig file if present.
3. Use a join token, if present.
4. Bail out with an error.

Fixes #2374.

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