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>
yes I knew about 'ceph daemon' but somehow I can't get this command work with client.admin... That's why I ended up with ceph --admin-daemon :)