Move MQTT publisher to fledge

[JakobVogelsang]

The Fledge organization has allowed us to put the efforts we did on the MQTT publisher south plugin at the moment, hosted here https://github.com/mz-automation/fledge-mqtt-publisher-south. We are waiting to one of the maintainers open a new repository for us. We then can push the code following the contributing guidelines of the fledge project.

Requirements:

• push code hosted https://github.com/mz-automation/fledge-mqtt-publisher-south to fledge repository following the guidelines
• properly document the plugin