start subscriber

def on_start(self):
        """
        start subscriber
        """
        LOGGER.debug("zeromq.Subscriber.on_start")
        try:
            self.zmqsocket.connect(self.zmqbind_url)
            self.zmqsocket.setsockopt_string(zmq.SUBSCRIBE, self.zmqtopic)
        except Exception as e:
            LOGGER.error("error while subscribing ! " + e.__cause__)
            raise e
        self.subscriber = threading.Thread(target=self.run, name=self.subscriber_name)
        self.subscriber.start()