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>
http://docs.openstack.org/trunk/config-reference/content/section_glance-api.conf.html suggests glance.store.rbd.Store ?