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>
maybe make the example match the default format, like /var/run/ceph/ceph-client.cinder.19195.32310016.asok