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>
yeah, it's the address of the ceph context. The metavar $cctid was added in master and backported to firefly and giant, but not in point releases yet