compute: allow more granularity on hypervisor nodes

- Allow to enable / disable RBD storage for:
  * Instances & Volumes
  * Volumes only (instances will be stored locally then)
- Deprecate has_ceph param with backward compatibility
- If RBD backend for instances is enabled on Red Hat, fail with a
  message.

Closes-bug #486
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #487 from enovance/bug/486/emilien

compute: allow more granularity on hypervisor nodes