Models and migrations for group level clusters

What does this MR do?

Adds ability for create group level clusters in models. This MR + https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22438 is necessary for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22450

• Model - cluster_groups
• Update clusters model
  • validation to prevent mixing of group and project association
  • enforce validation cluster_type presence, on: :create
  • back-populate cluster_type enum
  • null: false in future release - https://gitlab.com/gitlab-org/gitlab-ce/issues/53026

What are the relevant issue numbers?

#34758 (closed)

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