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 
Volume plugins are not committed yet.

TL;DR: volume plugins need to know about where to write files, such as the empty directory foir EmptyDir, but we don;t want them to know all about kubelets.  These let me define a simple interface that publishes various Get*Dir() methods.  Commented