compute: RBD backend support

Since Havana, Nova is able to store VMs on RBD backend (ceph).

Change-Id: I17890a3e9e49572d989ad01383417b7256ef7e14
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit 076d415ba560206ef1299b090c4eb474ffc2bdc4)
Merge "compute: RBD backend support" into stable/havana