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

app/models/clusters/platforms/kubernetes.rb

@@ -69,7 +69,7 @@ def actual_namespace
         end
       end
 
-      def predefined_variables_for_project(project:)
+      def predefined_variables(project:)
         Gitlab::Ci::Variables::Collection.new.tap do |variables|
           variables.append(key: 'KUBE_URL', value: api_url)
 
@@ -79,9 +79,8 @@ def predefined_variables_for_project(project:)
               .append(key: 'KUBE_CA_PEM_FILE', value: ca_pem, file: true)
           end
 
-          if project.kubernetes_namespace.present?
-            variables
-              .concat(project.kubernetes_namespace.predefined_variables)
+          if kubernetes_namespace = cluster.kubernetes_namespaces.find_by(project: project)
+            variables.concat(kubernetes_namespace.predefined_variables)
           else
             # From 11.5, every Clusters::Project should have at least one
             # Clusters::KubernetesNamespace, so once migration has been completed,