Refactor rpc backend vhost creation

The creation of the cellsv1 rpc vhost was buried in the restart function,
which makes it hard to extend. This breaks it out into a helper method
and moves the conditional logic into the nova module itself.

Change-Id: Ib0e377aabe45c27bb6ce59ca275ce73085e8b9d2
diff --git a/lib/nova b/lib/nova
index 4c26420..f5ab201 100644
--- a/lib/nova
+++ b/lib/nova
@@ -644,6 +644,7 @@
     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`
+        rpc_backend_add_vhost child_cell
         iniset_rpc_backend nova $NOVA_CELLS_CONF DEFAULT child_cell
         iniset $NOVA_CELLS_CONF DEFAULT dhcpbridge_flagfile $NOVA_CELLS_CONF
         iniset $NOVA_CELLS_CONF cells enable True