fix issue #121, unload plugins, stop/start gproc app when clustering...

apps/emqttd/src/emqttd_ctl.erl

@@ -75,12 +75,21 @@ cluster([SNode]) ->
     Node = node_name(SNode),
     case net_adm:ping(Node) of
     pong ->
+        case emqttd:is_running(Node) of
+            true ->
+                %%TODO: should not unload here.
+                emqttd:unload_all_plugins(),
                 application:stop(emqttd),
                 application:stop(esockd),
+                application:stop(gproc),
                 emqttd_mnesia:cluster(Node),
+                application:start(gproc),
                 application:start(esockd),
                 application:start(emqttd),
                 ?PRINT("cluster with ~p successfully.~n", [Node]);
+            false ->
+                ?PRINT("emqttd is not running on ~p~n", [Node])
+        end;
     pang ->
         ?PRINT("failed to connect to ~p~n", [Node])
     end.