Use field values from config.json and collect from request

def __collect_fields(self):
        """ Use field values from config.json and collect from request """
        form = FormData()
        form.add_field(self.__username_field, required=True,
                       error=self.__username_error)
        form.add_field(self.__password_field, required=True,
                       error=self.__password_error)
        form.parse()
        self.username = form.values[self.__username_field]
        self.password = form.values[self.__password_field]
        return