Kubelet directory-name builder funcs

This is shake-out from playing with volumes.  Not sure if the rollback stuff is right - we do not clean up other stuff if a pod fails to start...

@dchen1107 
you a) have to already have named your pod "pods", b) this is UID, not name, c) it will confuse things, but you th euser don't get access to it, d) transitional.  Better ideas?