EE port for: 48098 Mutual auth cluster applications

What does this MR do?

This is an EE port for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20928. The GetCommand and UpgradeCommand in EE only were broken by this refactoring to BaseCommand

1. Proposal to resolve code for Gitlab::Kubernetes::Helm::UpgradeCommand broken by underlying change to Gitlab::Kubernetes::Helm::BaseCommand (CE).
2. Manual EE MR for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20801

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

EE port for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20928.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • [-] Has been reviewed by a UX Designer
  • [-] Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• [-] Internationalization required/considered
• [-] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Manual QA To Be Done

• We need to test (manually) that the UpgradeCommand still works with this new mutual auth logic
• We need to test for backward compatibility of UpgradeCommand:
  • Checkout master (ie. code before mutual auth feature was added)
  • Restart gdk
  • Install helm tiller and prometheus
  • Checkout this code (ie. with mutual auth code added)
  • Restart gdk
  • Upgrade prometheus
  • Check upgrade succeeded by looking at pods in K8s dashboard
  • Check monitoring tab is still working