Use cinder volumes as a transparent alternative to lvm,rdb,qcow2,etc

Right now, nova support 4 backends for image storage: raw, qcow2, lvm and rdb. NFS could be configured by mounting it over /var/lib/nova/instances.

This blueprints add a fourth backend: cinder. In a modern nova setup, cinder is likely to already be set up. Using cinder as the image backend allows reduction of the number of storage locations for VMs, while significantly increasing the available options (from a nova perspective).