add RFC952 validation to labels

Add label validation of labels to  pods, services, relicationControllers, and replControllerState

Fixes #1297
I'm hoping that Docker will adopt labels, which says to me that namespacing labels IS important.

We can do it as a followup.  I would be OK with changing it to [ DNS1123_SUBDOMAIN "/" ] DNS952_LABEL, so it is a strict extension