Automatic merge from submit-queue

federation: Updating federation-controller-manager to use secret to get federation-apiserver's kubeconfig

Fixing the credentials problem: https://github.com/kubernetes/kubernetes/issues/26762#issuecomment-223690990.

Admin will create a secret with the name "federation-apiserver-secret" in the k8s cluster hosting the federation control plane. This secret will contain the kubeconfig to access federation-apiserver.
federation-controller-manager will use this secret to contact the federation-apiserver.
This flow is same as the one used by all federation-controllers to contact k8s apiservers that are part of the federation.

cc @kubernetes/sig-cluster-federation @lavalamp @erictune @colhom