Hitting the off button turns off all thermostats

[foxy82]

Hitting the off button on the lovelace card turns off all thermostats not the one selected.

This is probably a limitation of the warmup API which using the android app has a "Heating System Off" which is what we must be calling and it turns all thermostats off.

Proposal

1. Create as service for this "all_off" function

2. Look if we can find a way to turn an individual thermostat off via the API if that isn't possible I suggest we set a fixed temp of "min_temp" (for me 5 oC) is the best version of OFF we can provide.

[MathieuHd]

Fix proposed in #84 (comment)