Rationalize use of namespace/name.

Address @thockin comment from #1297 about namespace/name being consistent across objects, resources, and labels.

Confined k8s resources to have names which are DNS_SUBDOMAIN format, rather that path.  Since we only have two resources so far, it seemed safe to restrict this space a bit.  FYI @johnwilkes 
 
Also added text to labels doc describing their format.  FYI @bgrant0607 

Also FYI @derekwaynecarr 
Hmm.  I agree.  Part of the problem is overloading of the word "key": a "label key" should not contain a "key" ;-).  

On a vaguely related note, I don't see why the non-namespace part should look like a subdomain. I would suggest conformance to a DNS_LABEL format.