hypervisor: ensure RBD can't be enabled on RHEL

RHEL does not support RBD backend for VMs storage.
We need to ensure that nova will never be configured with RBD
parameters, even if has_ceph is true on Red Hat hypervisor nodes.

Also add more unit tests.
Merge pull request #438 from enovance/rh-compute-ceph

hypervisor: ensure RBD can't be enabled on RHEL