Add support for environment variable to load the configuration

Hi,

A little something I'm using for some weeks with docker-compose. There's no more need to have a volume so it's easier to deploy. I'm using a docker-compose.yml like this one :

version: '3'

services:
  aqara:
    image: monster1025/aqara-mqtt:1-armhf
    restart: always
    environment:
      AQARA_MQTT_CONFIG: |-
        mqtt:
          server: 127.0.0.1
          port: 1883
          username: username
          password: pwd
          prefix: home

        gateway:
          password: 12345667
          unwanted_data_fix: False
          polling_interval: 2
          polling_models:
            - motionx

        sids:
          # motion
          158d0001292a42:
            model: motion
            name: salon

          # buttons
          158d000129f700:
            model: switch
            name: kitchen

          # gateway
          f0b429cc25e8:
            model: gateway
            name: main

monster1025 / aqara-mqtt

Add support for environment variable to load the configuration #20