XD-1378: Refactor MQTT module options

Please note that I did not manage to run the shell script test for MQTT successfully, even though in theory, no defaults have changed.

Moreover, if I 

1. start a server
2. run the shell script
    1. it fails but leaves the stream "mqtt | file" in place
3. then create an additional stream as "http | mqtt" using the XD shell
4. `http post --data helloMQTT`, the payload *does* end up in the `/tmp/xdtest/basic/mqttSourceTest.out` file