Check for overlapping replication controller.

Fix #2210  @erictune 
I'm disappointed in GitHub for not comparing patch sets and automatically cloning the conments....

> On Nov 18, 2014, at 7:36 PM, bgrant0607 <notifications@github.com> wrote:
> 
> In pkg/api/validation/validation.go:
> 
> > @@ -454,6 +459,17 @@ func ValidateReplicationController(controller *api.ReplicationController) errs.V
> >  	}
> >  	allErrs = append(allErrs, ValidateReplicationControllerSpec(&controller.Spec).Prefix("spec")...)
> >  	allErrs = append(allErrs, validateLabels(controller.Labels)...)
> My bad. I wasn't aware of the earlier attempt. I was thinking purely about addressing the potential for user confusion. Will take a look.
> 
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