convinent access method for list.

        service_name describes the endpoint to call
        the `list` function on.

        images.list or apks.list.

def list(self, service_name, **params):
        """
        convinent access method for list.

        service_name describes the endpoint to call
        the `list` function on.

        images.list or apks.list.
        """
        result = self._invoke_call(service_name, 'list', **params)
        if result is not None:
            return result.get(service_name, list())
        return list()