Automatic merge from submit-queue

make the resource prefix in etcd configurable for cohabitation

This looks big, its not as bad as it seems.

When you have different resources cohabiting, the resource name used for the etcd directory needs to be configurable.  HPA in two different groups worked fine before.  Now we're looking at something like RC<->RS.  They normally store into two different etcd directories.  This code allows them to be configured to store into the same location.

To maintain consistency across all resources, I allowed the `StorageFactory` to indicate which `ResourcePrefix` should be used inside `RESTOptions` which already contains storage information.

@lavalamp affects cohabitation.
@smarterclayton @mfojtik prereq for our rc<->rs and d<->dc story.