Add Cinder backend to image store

Cinder is an potiential backend in Glance.
This patchs aims to:
- create glance::backend::cinder
- improve spec_helper using shared_examples

Change-Id: I15e5de8281ff2e60465fedda97d9c9ccf808386c
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>