Revisit identifiers spec

Partially addresses #1135.

This is the result of discussions in the past few days about imprecision and confusion. Hopefully simpler. There are a few FIXME comments in the doc - discuss.
Kubelet HAS to have some concept of namespacing, or else we can not differentiate a user pod named "cadvisor" from the config-file carried pod named "cadvisor".  At your request, I buried this into kubelet, but I don't think it can go away.  If the "other" namespacing proposal imposes new fields, we can hoist it back up to the API.