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 
I agree on conflation of namespaces.  Suggest "not specified" means "the default namespace", with a caveat that any system-defined keys will never use the default namespace.

I'm guessing that the use of DNS_SUBDOMAIN here is to unify this with other namespace/name pairs, of which at least object name may be a DNS_SUBDOMAIN.