doc: update RBD for Juno

This commit introduces some updates for the OpenStack Juno release. New
flags have been added, many trailing spaces were removed and a new
recommendation for Glance cache management has been added too.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #2835 from leseb/doc-rbd-juno

doc: update RBD for Juno

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>