function publish(topic, message = undefined) {
  if (this.mqttClient) {
    logger.trace(`Publish MQTT message over ${topic}`);
    this.mqttClient.publish(topic, message, undefined, (err) => {
      if (err) {
        logger.warn(`MQTT - Error publishing to ${topic} : ${err}`);
      }
    });
  }
}
@description Publish a message on a topic.
@param {string} topic - Topic to publish to.
@param {any} message - Message to send.
@example
publish('topic/to/send/to', 'MyMessage');