Automatic merge from submit-queue

Support for preexisting replicas and estimated capacity in federated replicaset controller

With this PR the planer will be able to:
* Keep already existing replicas in their current clusters if rebalance = false and min/max boundaries are met.

* Limit the number of replicas in a cluster to the level that was measured by the count of running and unschedulable pods. And provide an estimate how much more pods would be nice to put in a cluster so that if they are scheduled we will be closer to the desired layout or to schedule the desired number of replicas at all.

cc: @quinton-hoole @jianhuiz @wojtek-t @kubernetes/sig-cluster-federation