Replace deprecated config option [DEFAULT].rabbit_vritual_host

The test job "gate-tempest-dsvm-cells" uses the Nova cells concept.
This triggered a deprecation warning:

    WARNING oslo_config.cfg [...]
    Option "rabbit_virtual_host" from group "DEFAULT" is deprecated.
    Use option "rabbit_virtual_host" from group "oslo_messaging_rabbit".

This change removes that warning.

Change-Id: Ieaf437ecbf58edb8994f6afcb0ac2afcd5585a1e
diff --git a/lib/nova b/lib/nova
index ad72fc3..8823e1d 100644
--- a/lib/nova
+++ b/lib/nova
@@ -648,7 +648,7 @@
     if is_service_enabled n-cell; then
         cp $NOVA_CONF $NOVA_CELLS_CONF
         iniset $NOVA_CELLS_CONF database connection `database_connection_url $NOVA_CELLS_DB`
-        iniset $NOVA_CELLS_CONF DEFAULT rabbit_virtual_host child_cell
+        iniset $NOVA_CELLS_CONF oslo_messaging_rabbit rabbit_virtual_host child_cell
         iniset $NOVA_CELLS_CONF DEFAULT dhcpbridge_flagfile $NOVA_CELLS_CONF
         iniset $NOVA_CELLS_CONF cells enable True
         iniset $NOVA_CELLS_CONF cells cell_type compute