Domain was introduced on 11.8 and was not included on the
Project Cluster API. With this change user will be able to include
domain when adding and updating a cluster. Domain will also be included
on the GET calls.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/59441
Modifies authorize! method to accept a third param, and then use it in
combination with 'add_cluster' policy to appropriately restrict adding
multiple clusters
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56110
Adds the following initial CRUD endpoints for Clusters API:
- GET list of clusters
- GET specific cluster
- POST add existing cluster (mimic of "Add cluster")
- PUT update cluser
- DELETE destroy cluster
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/40473
