Bug Report - mqtt env variables not correct

[ms81229]

Describe the bug
The environment variables for username and password in the teslamate app are MQTT_USERNAME and MQTT_PASSWORD -- not MQTT_USER and MQTT_PASS. Unfortunately the teslamate app doesn't provide any validation of the environment variables so this is really hard to track down. I recommend updating the documentation so people don't cut and paste the wrong values into their use of teslamate.

Steps to reproduce

1. set env variable to connect to a MQTT server that does not use anonymous login
2. teslamate docker crashes

Screenshots