Check for overlapping replication controller.

Fix #2210  @erictune 
Ok, just took a look at how we organize labels, I think we should only check superset here. Just to make sure.  If existing rc targets on Pods "a=b,c=d", then rc with selector "a=b" will be rejected by apiserver while rc "a=b,c=d,e=f" will be created.

@bgrant0607 The lister will only list namespaced controllers (with ctx param), isn't it already restriced by namespace?