Kubernetes private credentials

Merged Kamil Trzcińśki requested to merge kubernetes-private-credentials into master

This adds an image pull secrets for Docker Registry credentials that are passed with GitLab Runner payload.

Does this MR meet the acceptance criteria?

  • Documentation created/updated
  • Tests
    • Added for this feature/bug
    • All builds are passing
  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/27763