DBaaS: Ability to force unregister Kubernetes cluster

Use cases: 

1. User deleted kubernetes cluster manually but forgot to unregister in PMM.  He should be able to force unregister kubernetes cluster.
2. User wants to unregister kubernetes cluster with existed DB clusters. DB Clusters shouldn't be deleted, but kubernetes cluster should be. 

 

UI:

Modal window with "force" checkbox to be checked to execute force un-registration  

 

Acceptance Criteria:

• User is able to execute Unregister action in Force mode 
• Force mode is not checked by default  
• Using force mode K8s Cluster is remove  from the list even if there were some errors  during this process 

How to test:

• Setup kubernetes cluster (minikube)
• Register kubernetes cluster (via UI)
• Stop or delete the physical kubernetes cluster (with a `minikube delete` command)
• Unregister kubernetes cluster (via UI)
• Kubernetes cluster should be removed (UI)