Define libvirt_group for RHEL family

We use the ceilometer::params::libvirt_group in the ceilometer agents
but since we do not define it for the RHEL family, it results in an
warning about an undefined variable. This change defines it as undef to
cleanup the warnings.

Change-Id: I6d768df0b7f3353171e90ca30b7f0d94fdc5d8dc

manifests/params.pp

@@ -31,6 +31,7 @@ class ceilometer::params {
       $agent_notification_service_name = 'openstack-ceilometer-notification'
       $ceilometer_wsgi_script_path     = '/var/www/cgi-bin/ceilometer'
       $ceilometer_wsgi_script_source   = '/usr/lib/python2.7/site-packages/ceilometer/api/app.wsgi'
+      $libvirt_group                   = undef
     }
     'Debian': {
       # package names