loxi-prep: rename action_list.add to append

For consistency with the list type.
diff --git a/tests/flow_expire.py b/tests/flow_expire.py
index ab19ec1..70ab10e 100644
--- a/tests/flow_expire.py
+++ b/tests/flow_expire.py
@@ -62,7 +62,7 @@
         request.idle_timeout = 1
         request.flags |= ofp.OFPFF_SEND_FLOW_REM
         act.port = egress_port
-        request.actions.add(act)
+        request.actions.append(act)
         
         logging.info("Inserting flow")
         self.controller.message_send(request)