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 just went and checked the linked-to page (identifiers.md).  As a result, I now think "name" should probably be "label", given that "name" is defined to be something else on that page.  This line should thus read: 

A resource typename has the form: `[namespace "/"] label`.  Namespace is a string that conforms to DNS_SUBDOMAIN format, and label is one that conforms to DNS_LABEL format (see [docs/identifiers.md](https://github.com/GoogleCloudPlatform/kubernetes/blob/master/docs/identifiers.md)).