Incorporates Kubernetes Namespace into Cluster's flow

What does this MR do?

Incorporates Clusters::KubernetesNamespace into clusters flow:

• Create the namespace/service account in GCP Kubernetes, when creating a cluster
• Create the namespace/service account in GCP Kubernetes, when adding a cluster
• Create the namespace/service account in GCP Kubernetes, when modifying namespace
• Replace KUBE_TOKEN and KUBECONFIG (passed to CI) with new credentials

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/51716

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

Manual QA

ABAC Cluster

• Add a Cluster
• Check namespace/service account were created on kubernetes side
• Install applications
• Run AutoDevOps pipeline

RBAC Cluster

Creating cluster

• Create Cluster
• Check namespace/service account were created on kubernetes side
• Install applications
• Run AutoDevOps pipeline

Auto DevOps for ABAC

• Switch to master branch
• Create a cluster ABAC
• Switch to this branch, restart GDK
• Run Auto DevOps pipeline

Auto DevOps for RBAC

• Switch to master branch
• Create a cluster
• Switch to this branch, restart GDK
• Run Auto DevOps pipeline

.gitlab/issue_templates/Security developer workflow.md

@@ -16,7 +16,6 @@ Set the title to: `[Security] Description of the original issue`
 - [ ] Add a link to the MR to the [links section](#links)
 - [ ] Add a link to an EE MR if required
 - [ ] Make sure the MR remains in-progress and gets approved after the review cycle, **but never merged**.
-- [ ] Assign the MR to a RM once is reviewed and ready to be merged. Check the [RM list] to see who to ping.
 
 #### Backports
 
@@ -26,7 +25,8 @@ Set the title to: `[Security] Description of the original issue`
     - [ ] Create the branch `security-X-Y` from `X-Y-stable` if it doesn't exist (and make sure it's up to date with stable)
     - [ ] Create each MR targetting the security branch `security-X-Y`
     - [ ] Add the ~security label and prefix with the version `WIP: [X.Y]` the title of the MR
-- [ ] Make sure all MRs have a link in the [links section](#links) and are assigned to a Release Manager.
+- [ ] Add the ~"Merge into Security" label to all of the MRs.
+- [ ] Make sure all MRs have a link in the [links section](#links)
 
 [secpick documentation]: https://gitlab.com/gitlab-org/release/docs/blob/master/general/security/developer.md#secpick-script