Remove Identity Cluster relationship from cluster service in case of requesting delete for openshift external token

[dipak-pawar]

If a user doesn't want to have his account link to some particular cluster then he might want to remove it from UI. For that, we will need to remove the token from Auth and for that cluster/identity from cluster service.

We have an endpoint to remove external_token. Let's update it to remove identity cluster relationship. So user removes it from UI. UI calls Auth to remove the token. Auth calls Cluster service to remove Identity_cluster.