EE: User can create a group level cluster and install applications

⚠ NOTE: the target branch of ee-top_level_clusters_controller is temporary until that branch is merged

What does this MR do?

EE counterpart to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22450 and implements ability to create multiple group level clusters (EE GitLab Premium )

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/34758

Tasks

• Fix ClustersHelper to properly override with a EE module
• group.feature_available?(:multiple_clusters) - what plan ?
• CreateService#can_create_cluster?
• ClustersHelper#has_multiple_clusters?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?