Check for overlapping replication controller.

Fix #2210  @erictune 
Yes, that was the intent of the filed issue. We should check whether any other controllers target the same pods as this controller. With the current exact-match selectors, this means one of the selectors is a subset of the other. 

If we wanted to be even more paranoid, we could also outlaw selectors without at least one key in common, but I'd at most put a TODO for that right now.

And, all of these checks should be restricted by namespace.