doc: enable RBD cache and socket on OpenStack deployments

Enabling the RBD cache improves sequential IOs and the socket helps a
lot while troubleshooting. These 2 items are considered as best
practice for OpenStack deployments with Ceph.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Josh and I talked about just enabling these sockets by default for hammer.  We'll want to doucment for firefly, but lets' make the file name line up.  Probably /var/run/ceph/$cluster-$type.$id.$pid.$cct.asok?