doc: update the OpenStack glance configuration

Juno is now using a section for configuring the stores, so let's use it.
Put spaces between flag and value for clarity and readibility.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #2917 from leseb/rbd-update-glance-config

doc: update the OpenStack glance configuration

Reviewed-by: Loic Dachary <ldachary@redhat.com>