Kubernetes deployments on new clusters will now have
a separate namespace per project environment, instead
of sharing a single namespace for the project.

Behaviour of existing clusters is unchanged.

All new functionality is controlled by the
:kubernetes_namespace_per_environment feature flag,
which is safe to enable/disable at any time.

Using the sed script from
https://gitlab.com/gitlab-org/gitlab-ce/issues/59758

Also means we don't have to resort to an update statement to set parent
for child groups who also have clusters.

This is much shorter than

```
create(:cluster, :provided_by_gcp, :group, groups: [group])
```

When this option is enabled, GitLab will create namespaces and service
accounts as usual. When disabled, GitLab wont create any project
specific kubernetes resources

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56557

Adds boolean field 'managed' to clusters table. This new column will be
used to distinguish those clusters that are automatically managed by
gitlab.

Needed for https://gitlab.com/gitlab-org/gitlab-ce/issues/56557

Only create an associated project or group if there
were none already specified.

This is the 1st step for moving Auto DevOps domain into cluster
settings, whether is project or group.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/52363

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.