Handle envirement var in alertmanager/config.yml

[KoVaLsKy2010]

handle envirement vars
*.yml files dose not handle envirement from docker

Environment

• Alertmanager configuration file NOW:

 receivers:
      - name: 'telegram'
        telegram_configs:
          - send_resolved: true
            api_url: 'https://api.telegram.org'
            bot_token: '123456:ASDFGH-:LKHHGFGD-WERWERWER'
            chat_id: 12345
            parse_mode: 'Markdown'

For example

receivers:
      - name: 'telegram'
        telegram_configs:
          - send_resolved: true
            api_url: 'https://api.telegram.org'
            bot_token: ${TELEGRAM_TOKEN}
            chat_id: ${TELEGRAM_ADMIN}
            parse_mode: 'Markdown'

docker-compose.yml

alertmanager:
        image: prom/alertmanager
        ports:
            - 9093:9093
        environment:
            - TELEGRAM_ADMIN=12345
            - TELEGRAM_TOKEN='123456:ASDFGH-:LKHHGFGD-WERWERWER'

[splitt3r]

Duplicate of #504

#2498 tries to solve the problem with secret files