The NFS implementation based off cinder_nfs_client
does not support per cinder backend configuration,
separating NFS mounts per backend.
This fix removes cinder_nfs_client in favor of
fully supporting multi cinder backends over the
cinder_backends stanza.
Closes-Bug #1657203
Change-Id: I817ff372c2d58e999a89496fce3d25dc617216f0