nova-manage network commands only when n-net enabled

This patch checks if n-net is enabled rather than any nova service
before creating networks through nova.  Fixes bug 1073313

Change-Id: I8810737ddbb26a5e281060f5395cfad5d186b6d3
diff --git a/stack.sh b/stack.sh
index 366d752..7df32da 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1916,7 +1916,7 @@
         fi
    fi
 
-elif is_service_enabled $DATABASE_BACKENDS && is_service_enabled nova; then
+elif is_service_enabled $DATABASE_BACKENDS && is_service_enabled n-net; then
     # Create a small network
     $NOVA_BIN_DIR/nova-manage network create "$PRIVATE_NETWORK_NAME" $FIXED_RANGE 1 $FIXED_NETWORK_SIZE $NETWORK_CREATE_ARGS