Publish a message for all turrets

        :param dict message: message to send to turrets
        :pram str channel: channel to send message, default to empty string

def publish(self, message, channel=None):
        """Publish a message for all turrets

        :param dict message: message to send to turrets
        :pram str channel: channel to send message, default to empty string
        """
        if not self.master:
            return
        channel = channel or ''
        data = json.dumps(message)
        self.publisher.send_string("%s %s" % (channel, data))