Add Mosquitto service

[amotl]

Dear @vangheem and @masipcat,

first things first: Thanks for conceiving and maintaining this valuable pytest fixture module.

We needed the Mosquitto service for running integration tests against and added a [WIP] at [1]. It should be improved by also integrating the MQTT client library paho-mqtt.

Unless we will be able to submit a pull request, it might serve as a starter for others.

With kind regards,
Andreas.

[1] https://github.com/hiveeyes/terkin-datalogger/blob/ccaa56d6/test/util/mosquitto.py