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

@@ -70,14 +70,9 @@ def actual_namespace
       end
 
       def predefined_variables
-        config = YAML.dump(kubeconfig)
-
         Gitlab::Ci::Variables::Collection.new.tap do |variables|
           variables
             .append(key: 'KUBE_URL', value: api_url)
-            .append(key: 'KUBE_TOKEN', value: token, public: false)
-            .append(key: 'KUBE_NAMESPACE', value: actual_namespace)
-            .append(key: 'KUBECONFIG', value: config, public: false, file: true)
 
           if ca_pem.present?
             variables
@@ -118,11 +113,11 @@ def kubeconfig
         to_kubeconfig(
           url: api_url,
           namespace: actual_namespace,
-          token: fetch_token,
+          token: default_service_account_token,
           ca_pem: ca_pem)
       end
 
-      def fetch_token
+      def default_service_account_token
         kubernetes_namespace&.service_account_token.presence || token
       end