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.

manifests/compute/hypervisor.pp

@@ -102,6 +102,13 @@ Host *
       mode   => '0644',
       notify => Service['libvirtd']
     }
+    # Nova support for RBD backend is not supported in Red Hat packages
+    if $has_ceph {
+      warning('Red Hat does not support RBD backend for VMs.')
+    }
+    $has_ceph_real = false
+  } else {
+    $has_ceph_real = $has_ceph
   }
 
   if $::operatingsystem == 'Ubuntu' {
@@ -118,7 +125,7 @@ Host *
 
   class { 'nova::compute::neutron': }
 
-  if $has_ceph {
+  if $has_ceph_real {
 
     $libvirt_disk_cachemodes_real = ['network=writeback']
     include 'cloud::storage::rbd'