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

@@ -6,6 +6,7 @@ class Kubernetes < ActiveRecord::Base
       include Gitlab::Kubernetes
       include ReactiveCaching
       include EnumWithNil
+      include AfterCommitQueue
 
       self.table_name = 'cluster_platforms_kubernetes'
       self.reactive_cache_key = ->(kubernetes) { [kubernetes.class.model_name.singular, kubernetes.id] }
@@ -39,6 +40,7 @@ class Kubernetes < ActiveRecord::Base
       validate :prevent_modification, on: :update
 
       after_save :clear_reactive_cache!
+      after_update :update_kubernetes_namespace
 
       alias_attribute :ca_pem, :ca_cert
 
@@ -196,6 +198,14 @@ def prevent_modification
 
         true
       end
+
+      def update_kubernetes_namespace
+        return unless namespace_changed?
+
+        run_after_commit do
+          ClusterPlatformConfigureWorker.perform_async(cluster.id)
+        end
+      end
     end
   end
 end