Allow NFS configuration of storage backend for Nova.
This way the instances files will be stored on a shared
NFS storage.
Implements: bp tripleo-nova-nfs
Depends-On: Id15aec6324814a871e87f19f24999b0e3b8a8f05
Change-Id: Ie4fe217bd119b638f42c682d21572547f02f17b2