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/services/clusters/kubernetes/configure_service.rb

@@ -9,31 +9,28 @@ class ConfigureService
 
       def initialize(platform)
         @platform = platform
-        @cluster_project = platform.cluster_project
       end
 
       def execute
-        return unless cluster_project
+        return unless platform.cluster_project
 
-        kubernetes_namespace = cluster_project.kubernetes_namespaces.create!
-        kubernetes_namespace
-
-        create_kubernetes_namespace(kubernetes_namespace.namespace)
-        create_kubernetes_service_account(kubernetes_namespace.service_account_name)
-      rescue ActiveRecord::RecordInvalid => e
-
-        # To do: Create service account
+        create_kubernetes_namespace
+        create_kubernetes_service_account
       end
 
       private
 
-      def create_kubernetes_namespace(namespace_name)
-        Gitlab::Kubernetes::Namespace.new(namespace_name, platform.kubeclient).ensure_exists!
+      def create_kubernetes_namespace
+        Gitlab::Kubernetes::Namespace.new(kubernetes_namespace.namespace_name, platform.kubeclient).ensure_exists!
       end
 
-      def create_kubernetes_service_account(service_account_name)
+      def create_kubernetes_service_account
         # To do
       end
+
+      def kubernetes_namespace
+        @kubernetes_namespace ||= platform.cluster_project.kubernetes_namespace
+      end
     end
   end
 end