add RFC952 validation to labels

Add label validation of labels to  pods, services, relicationControllers, and replControllerState

Fixes #1297
I don't think we want '.' here.  Less formats is better.  952Identifier (should be "Name" properly) is a strict subset of the main DNSLabel type.  Let's leave this one as a simple dns-compatible name token - no dots, unless you have a use case that needs dots in label keys?