Adds NFS backend support for cinder-volume

This patch installs nfs-utils, creates a directory for mounting
shares, adds configuration and sets up the shares file if
metadata keys "include_nfs_backend: true" and "nfs_shares" are
present.

See https://review.openstack.org/#/c/75017/ for template info.

Change-Id: Id29015e6b2a97ee8a13049eb7d9161781b415be4
Merge "Adds NFS backend support for cinder-volume"