Honor API port number in kubeconfig admin subcommand

[twz123]

Description

The subcommand was not properly replacing the internal cluster URL with the external one with a non-default port for the API. Instead of doing a string replacement on the raw kubeconfig, parse it and do a structured replacement of the URL. Adjust the error message when the admin config is missing. Add some comments that give some hints on how the kubeconfig admin subcommand relates to the kubeconfig generation in certificates.go. Add some unit tests.

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