nova: enable RBD on RHEL 7

manifests/compute/hypervisor.pp

@@ -217,8 +217,8 @@ Host *
       mode   => '0644',
       notify => Service['libvirtd']
     }
-    # Nova support for RBD backend is not supported in Red Hat packages
-    if $has_ceph or $vm_rbd {
+    # Nova support for RBD backend is not supported in Red Hat 6.x packages
+    if $has_ceph or $vm_rbd and $::operatingsystemrelease < 7 {
       fail('Red Hat does not support RBD backend for VMs.')
     }
   }