Improve the MQTT interface: Use the Homie library/specification

[Drolla]

This is a remaining suggestion from @rkoshak taken over from #6 to use the Homie library/specification for the MQTT interface.

[rkoshak]

Here's an example which might make things easier for you.

https://github.com/rkoshak/sensorReporter/blob/main/mqtt/homie_conn.py

Note that it doesn't know ahead of time what topics need to be available so it is probably a little more complicated than you'd need to implement.