Split MQTT host section into class-provided and do-it-yourself brokers

[jfischer]

The text for the MQTT section is written as if the student is setting up their own broker. However, we provided a broker in the class. Many of the details were irrelevant or confusing (do they need Python3?) for the students. I think the best solution is to have two subsections: one if the class is providing a common broker and the other if you are doing it yourself.

See issue #4 for a example of the problems caused by having only one set of instructions.