Glance: add "local" backend

As an option, allow to configure Glance with "local" backend.
It is a option to use when not having Ceph in the environment.

For backward compatibility, we set 'rbd' as default backend.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #534 from enovance/glance-file

Glance: add "local" backend