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

@@ -47,7 +47,6 @@ class Kubernetes < ActiveRecord::Base
       delegate :managed?, to: :cluster, allow_nil: true
 
       delegate :cluster_project, to: :project, allow_nil: true
-      delegate :default_namespace, to: :cluster_project, allow_nil: true
 
       alias_method :active?, :enabled?
 
@@ -58,6 +57,12 @@ class Kubernetes < ActiveRecord::Base
       }
 
       def actual_namespace
+        cluster_project&.namespace || fallback_actual_namespace
+      end
+
+      # DEPRECATED
+      # To remove after migration of data to cluster_projects
+      def fallback_actual_namespace
         if namespace.present?
           namespace
         else
@@ -118,8 +123,16 @@ def kubeconfig
           ca_pem: ca_pem)
       end
 
+      # DEPRECATED
+      def default_namespace
+        return unless project
+
+        slug = "#{project.path}-#{project.id}".downcase
+        slug.gsub(/[^-a-z0-9]/, '-').gsub(/^-+/, '')
+      end
+
       def build_kube_client!(api_groups: ['api'], api_version: 'v1')
-        raise "Incomplete settings" unless api_url && actual_namespace
+        raise "Incomplete settings" unless api_url
 
         unless (username && password) || token
           raise "Either username/password or token is required to access API"