loxi-prep: rename action_list.add to append

For consistency with the list type.
diff --git a/tests/bsn_mirror.py b/tests/bsn_mirror.py
index 3c79ffd..79e9b26 100644
--- a/tests/bsn_mirror.py
+++ b/tests/bsn_mirror.py
@@ -143,9 +143,9 @@
         act3.port = ports[1]
         flow_mod = ofp.message.flow_mod()
         flow_mod.match = match
-        flow_mod.actions.add(act1)
-        flow_mod.actions.add(act2)
-        flow_mod.actions.add(act3)
+        flow_mod.actions.append(act1)
+        flow_mod.actions.append(act2)
+        flow_mod.actions.append(act3)
         delete_all_flows(self.controller)
         self.controller.message_send(flow_mod)
         do_barrier(self.controller)