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

db/migrate/20181009190428_create_clusters_kubernetes_namespaces.rb

 # frozen_string_literal: true
 
 class CreateClustersKubernetesNamespaces < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
-  disable_ddl_transaction!
-
   DOWNTIME = false
-  INDEX_NAME = 'kubernetes_namespaces_cluster_project_and_namespace'.freeze
+  INDEX_NAME = 'kubernetes_namespaces_cluster_project_and_namespace'
 
-  def up
-    create_table(:clusters_kubernetes_namespaces) do |t|
+  def change
+    create_table :clusters_kubernetes_namespaces do |t|
       t.references :cluster_project, null: false, index: true, foreign_key: { on_delete: :cascade }
 
       t.timestamps_with_timezone null: false
@@ -20,17 +16,8 @@ def up
       t.string :service_account_name
     end
 
-    add_concurrent_index(:clusters_kubernetes_namespaces,
-                         [:cluster_project_id, :namespace],
-                         unique: true,
-                         name: INDEX_NAME)
-  end
-
-  def down
-    drop_table :clusters_kubernetes_namespaces
-
-    if index_exists?(:cluster_kubernetes_namespaces, [:cluster_project_id, :namespace])
-      remove_concurrent_index :cluster_kubernetes_namespaces, [:cluster_project_id, :namespace]
-    end
+    add_index :clusters_kubernetes_namespaces, [:cluster_project_id, :namespace], 
+      unique: true, 
+      name: INDEX_NAME
   end
 end