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.
The current text says:
```
Kubelet further namespaces the pod name with information about the source of the pod.
```

The namespace concept shouldn't be specific to Kubelet. We should add namespaces generally. 

apiserver should fill it in if it's unspecified. Kubelet should expect the namespace to be populated in pods from the apiserver. 

The namespace may be populated in pods in files on minions, but if it's not, Kubelet can automatically fill it in.