Follow-up from "GitLab EKS Cluster backend services"

The following discussion from !16758 (merged) should be addressed:

• @mkozono started a discussion:

  It might be nice to rearrange this a little to avoid repeating the same conditionals, especially if we plan to add more later. E.g. could we reduce this to a single line provider.provision. And in WaitForClusterCreationWorker use provider.verify_provision? Or something to that extent.

  (Ok to leave for later if you prefer)