Return a value from kytos.json.

        Args:
            user (string): A Username.
            napp (string): A NApp name
            key (string): Key used to get the value within kytos.json.

        Returns:
            meta (object): Value stored in kytos.json.

def _get_napp_key(self, key, user=None, napp=None):
        """Return a value from kytos.json.

        Args:
            user (string): A Username.
            napp (string): A NApp name
            key (string): Key used to get the value within kytos.json.

        Returns:
            meta (object): Value stored in kytos.json.

        """
        if user is None:
            user = self.user
        if napp is None:
            napp = self.napp
        kytos_json = self._installed / user / napp / 'kytos.json'
        try:
            with kytos_json.open() as file_descriptor:
                meta = json.load(file_descriptor)
                return meta[key]
        except (FileNotFoundError, json.JSONDecodeError, KeyError):
            return ''