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
* Use [glance_store] section
* Update prefix when needed

Partial-Bug: #1391099

Change-Id: I096b16c33c493270422113e0107a100f967debc6
Merge "Update [glance_store] section in glance config for Juno"