Update RBD doc for OpenStack

Some updates from Icehouse, best practice configurations and
explanations.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>

Conflicts:
	doc/rbd/rbd-openstack.rst
Merge pull request #1905 from leseb/doc-rbd-icehouse

doc: rbd icehouse updated. Need to remove duplicate section.

Reviewed-by: John Wilkins <john.wilkins@inktank.com>