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>
hmm, we could do that, but i think the permissions are going to be 
problematic for /var/run/ceph until we create a 'ceph' user and that is 
way too big a change for a stable series