Automatic merge from submit-queue

federation: replacing string credentials field by secretRef

Fixes https://github.com/kubernetes/kubernetes/issues/25761

Replaced the string Credentials field in ClusterSpec by secretRef as discussed in https://github.com/kubernetes/kubernetes/issues/25761.
Also updated the clusterController to use this new secretRef field while creating a client to talk to a k8s cluster.

cc @lavalamp @kubernetes/sig-cluster-federation