allow base MQTT topic to be configurable (instead of hardcoded to teslamate/)

[sumnerboy12]

What would you like to be added:
Allow (optional) configuration of the base MQTT topic, instead of being hardcoded to teslamate/

Why is this needed:
I would like teslamate data to live under an existing base topic, along with all my other home automation data/feeds.

Additional context:

[sumnerboy12]

Just had a look at the code, one solution could be to remove teslamate from the hardcoded topic (line 79 in here) and just default namespace to teslamate if there is nothing explicitly defined in the env vars. This would keep the current behaviour (so long as namespace isn't set) and would mean no new config variables are needed.

This would break for anyone who currently uses namespace however, as they would need to prepend telsamate/ to keep current behaviour.