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/cluster.rb

@@ -19,6 +19,7 @@ class Cluster < ActiveRecord::Base
 
     has_many :cluster_projects, class_name: 'Clusters::Project'
     has_many :projects, through: :cluster_projects, class_name: '::Project'
+    has_one :cluster_project, -> { order(id: :desc) }, class_name: 'Clusters::Project'
 
     # we force autosave to happen when we save `Cluster` model
     has_one :provider_gcp, class_name: 'Clusters::Providers::Gcp', autosave: true