loxi-prep: rename action_list.add to append

For consistency with the list type.
diff --git a/tests/openflow_protocol_messages.py b/tests/openflow_protocol_messages.py
index 028d939..7ab49a2 100644
--- a/tests/openflow_protocol_messages.py
+++ b/tests/openflow_protocol_messages.py
@@ -221,7 +221,7 @@
                 msg.data = str(outpkt)
                 act = ofp.action.output()
                 act.port = dp_port
-                msg.actions.add(act)
+                msg.actions.append(act)
 
                 logging.info("PacketOut to: " + str(dp_port))
                 self.controller.message_send(msg)