follow comments test=develop

python/paddle_serving_server/web_service.py

@@ -122,7 +122,7 @@ class WebService(object):
                               processes=1)
 
     def get_app_instance(self):
-        return app_instance
+        return self.app_instance
 
     def preprocess(self, feed=[], fetch=[]):
         return feed, fetch

python/paddle_serving_server_gpu/web_service.py

@@ -171,7 +171,7 @@ class WebService(object):
                               processes=1)
 
     def get_app_instance(self):
-        return app_instance
+        return self.app_instance
 
     def preprocess(self, feed=[], fetch=[]):
         return feed, fetch