wazo-dird: some plugins don't accept unknown data.

When trying to add a Google directory under Nestbox / Directory on a fresh 19.13 install, the following error message is shown as the addition fails:

2019-10-02 07:44:17,097 [1683] (ERROR) (xivo.rest_api_helpers): Sent data is invalid: {'auth': {'certificate_path': ['Unknown field.']}} 2019-10-02 07:44:17,097 [1683] (INFO) (flask.app): response: (3.86.102.84) POST https://35.180.190.125/api/dird/0.1/backends/google/sources 400