function disconnect() {
  this.connected = false;

  if (this.mqttClient) {
    logger.debug(`Disconnecting existing MQTT server...`);
    this.mqttClient.end();
    this.mqttClient.removeAllListeners();
    this.mqttClient = null;
  } else {
    logger.debug('Not connected');
  }
}
@description Disconnect to all topics.
@example
disconnect();