callback template implementaion

app/app.py

@@ -1031,11 +1031,12 @@ class DeviceCallback(Resource):
   @app.param('url', 'Callback url', 'query')
   @app.response(HTTPStatus.OK, 'JSON data {"user":user,"state":state}')
   @app.response(HTTPStatus.UNAUTHORIZED, 'Authorization required')
-  async def get(self, device):
+  async def get(self, device, url):
     '''Returns device's callback url.
     '''
     if (device != request.device) and (not request.admin):
       abort(401)
+    url = request.args.get('url', None)
     if url is None:
       url = 'http://127.0.0.1:65535/status'
     return successCallbackURL(device, url)