Use the Kubernetes REST client for joining

[twz123]

Description

This makes the code a lot easier.

Augment the unit tests for the JoinClient with the happy cases, so that there's test coverage for this change:

• Capture new member node names in etcd controller. This moves the side effect of the os.Hostname() call out of the client, so that it doesn't influence the tests.
• Change the fake HTTP server to HTTPS, which will test the certificate handling properly.

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