Add MQTT support

[ezio-melotti]

The current version of SIMOC SAM (aka SIMOC live) relies on a custom implementation using websockets. This issue is about moving from that to a new implementation based on MQTT.

Plan

• [x] research/select a server/broker and client implementation
  • [x] broker: Mosquitto
  • open source, apt-getabile, no setup required
  • will replace sioserver.py
  • [x] client: paho-mqtt
  • Python package for MQTT clients
  • seems to be to most commonly used
• [x] test the setup locally:
  • [x] test with Mosquitto as a broker
  • [x] test with mosquitto_pub and mosquitto_sub as clients
  • [x] test with paho-mqtt as pub/sub clients
• [ ] update the code in the repo
  • [x] replace sioserver.py with Mosquitto
  • [x] update BaseSensor class to support MQTT through paho-mqtt
  • [ ] possibly add additional scripts to start/stop/manage/test the new setup

Open issues

1. Should this completely replace the socketio implementation, or be added as an alternative?
   • we should support multiple protocols (MQTT, socketio, DDS, etc.)
2. Should SIMOC (the web interface/dashboard) connect directly to the MQTT broker, or should we have an MQTT subscriber that keeps relying all messages to the dashboard through socketio?
   • there should be a subscriber in the backend that receives all data and sends them to the frontend through socketio
3. How much of the MQTT implementation do we want to expose to the dashboard? Should we have panels able to subscribe to certain MQTT topics?
   • the dashboard shouldn't know about the protocols used by the backend
4. What QoS levels should we use?
5. Do we want/need encryption?
6. How should we divide the topics?

See the MQTT support page for further instructions.