Add command to consul-k8s-control-plane: gossip-encryption-autogenerate

[t-eckert]

Changes proposed in this PR:

• Add a command to consul-k8s-control-plane called gossip-encryption-autogenerate
  This command takes in a namespace and a secret-name, checks for a kubeconfig being passed in or uses the default for the system. It checks if a secret already exists with the given secret-name and fails safely with code 0 if so. Otherwise it generates a 32 byte random string and stores it in the Kubernetes secret store.

How I've tested this PR:

• Unit tests

How I expect reviewers to test this PR:

• Unit tests
• Execute the command from ./control-plane with

  go run . gossip-encryption-autogenerate -namespace default -secret-name my-special-secret

  If successful, check the secret with

  kubectl get secrets my-special-secret -o yaml

  Check that the key value is a random base 64 encoded 32 byte string. Then run

  go run . gossip-encryption-autogenerate -namespace default -secret-name my-special-secret

  and observe that this returns "A Kubernetes secret with the name my-special-secret already exists." without overwriting the secret at that location.

Checklist:

• Tests added
• CHANGELOG entry added

  HashiCorp engineers only, community PRs should not add a changelog entry.
  Entries should use present tense (e.g. Add support for...)

[lkysow]

This is looking really good! I've got some suggestions to simplify things even further but overall things are excellent.

I'll probably not get back to this PR next week so feel free to merge once you've got the requisite approvals from others.

[t-eckert]

@lkysow, thank you for a great review! I have implemented all of your suggestions.

[kschoche]

This looks great, I left a few comments but overall it's looking good.

One thing I'd like to sort out before merging is being consistent on using k8s vs kubernetes, especially in error messages.
(bonus points if we can clean it up to be consistent on data structure names, flags and function names)

[ndhanushkodi]

This looks great, Thomas!!