add RFC952 validation to labels

Add label validation of labels to  pods, services, relicationControllers, and replControllerState

Fixes #1297
952 is the much stricter form of DNS label.   Do we need the strict form here?  And is DNS Label the correct input formatter?  And what about Annotations (new in v1beta3)?

Where are label/annotation keys to be used?  Will they be converted to environment variables?

I really do think they need to be namespaceable, and I don't think this regex is sufficient.  Would there be any harm in this becoming [DNS_SUBDOMAIN/]DNS_SUBDOMAIN - the same as resource types after #1323 goes through  The implication of leaving off the first part are different, but I can live with that.  @erictune