Extend RBAC to create service account restricted to project's namespace

Extend RBAC support to automatically create service account/role restricted to only the project's namespace and expose it as environment variable

Proposal:

• We need to create a ServiceAccount and a RoleBinding with edit access, under project's namespace.
• Replace KUBE_TOKEN and KUBECONFIG (passed to CI) with new credentials
• Remove the prompt that reads:

The default cluster configuration grants access to many functionalities needed to successfully build and deploy a containerised application. More information

Backend - To do

• Persist namespace information for cluster/project - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22011
• Create a Kubernetes service account under project's namespace
• Create a RoleBinding with edit access
• Replace KUBE_TOKEN and KUBECONFIG (passed to CI) with new credentials

List of MR's

1. Rails models (with respective migration) - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22404
2. Integration with the cluster/kubernetes services - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22011
3. Database background migration. - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22433
4. Introduce RoleBinding methods and class - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22524
5. Introduce new Kubernetes spec helpers - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22525
6. Changes to cluster views - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22550
7. Documentation - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22810/

Extra (optional /follow-ups):

1. Add cleanup worker that would remove degenerated namespaces (project_id=nil). https://gitlab.com/gitlab-org/gitlab-ce/issues/53591
2. Remove fallback_default_namespace after migration. https://gitlab.com/gitlab-org/gitlab-ce/issues/53585
3. Remove kubernetes_service integration. https://gitlab.com/gitlab-org/gitlab-ce/issues/39217
4. Remove extra branch on Clusters::Platforms::Kubernetes https://gitlab.com/gitlab-org/gitlab-ce/issues/53586
5. Compatibility with group clusters https://gitlab.com/gitlab-org/gitlab-ce/issues/53592