Fix Subscribers/Publishers(MQTT)

• make sure that the value published to/(read from) your middleware is read from/(written to) the Port the MQTTConnectionSymbol is attached to
• Subscriber
  • use variable port name instead of mqttIn(from here)
  • it must be possible to write to more than one port, since a MQTTConnectionSymbol could be added to each incomming Port. You might need to create multiple instances of Callback or multiple Callback classes.
• Publisher
  • use variable port name instead of mqttOut(from here)