Ability to force unregister kubernetes cluster - API

User Story: As a PMM Admin user I need to be able to unregister K8s Cluster from PMM, so that I can clean my env if there any problems with K8s 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. 

How to test:

• Setup kubernetes cluster

• Register kubernetes cluster

• Stop or delete kubernetes cluster

• Unregister kubernetes cluster

• Kubernetes cluster should be removed

Note:

Create separate UI task

Suggested implementation:

• Add new boolean field "force" to UnregisterKubernetesCluster API

• If force field is true don't check for DB clusters and unregister kubernetes cluster

• If force field is false keep current behaviour

• create API test for this endpoint with force param