Even though we currently only should have one group for a cluster, we
allow the flexibility to associate to other groups in the future.
This also matches the runner <=> groups association.
- Adds Cluster#first_group, aliased to Cluster#group. For the
conceivable future, a cluster will have at most one group.
- Prevent mixing of group and project clusters. If project type
clusters, it should only have projects assigned. Similarly with groups.
- Default cluster_type to :project_type. As it's very small table we can
set default and null: false in one release.
