Update [glance_store] section in glance config for Juno

Use new parameters for Juno like it's documented here:
http://docs.openstack.org/trunk/config-reference/content/glance-conf-changes-master.html
http://docs.openstack.org/trunk/config-reference/content/configuring-image-service-backends.html
* Use [glance_store] section
* Update prefix when needed

From Change-Id: I096b16c33c493270422113e0107a100f967debc6

Closes-bug: 1391475
Co-Authored-By: Emilien Macchi <emilien.macchi@enovance.com>

Change-Id: I5174e1b6a6d4ac978680325a5fad46001c5ca59f