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>
On that patch (https://review.openstack.org/#/c/121578/), Flavio Percoco was suggesting "rbd" only.