def define_retention(self, tsuid, retention_days):
        raise NotImplementedError
@property: list of tasks directly downstream
dataset/ETHPy150Open openstack/python-opentsdbclient/opentsdbclient/base.py/BaseOpenTSDBClient.define_retention--crossover with--dataset/ETHPy150Open airbnb/airflow/airflow/models.py/BaseOperator.downstream_list