Update install docs to match default NFS config
Update install docs to match default NFS config

nfs_shares_config default is "nfs_shares" but install doc instructed to
use "nfsshares". Following the documented instructions would work, but
we should use the default config value.

Closes-bug: #1716987

Change-Id: Ia5fb7a43cd47ed8fdd3e561affd9c3af95e6b40b
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

mahak/cinder,openstack/cinder,mahak/cinder,openstack/cinder