Use proxy_from_environment in Alertmanager

[TheoBrigitte]

Use the proxy_from_environment option in the Alertmanager configuration to make it read the proxy settings from environment variables (HTTP_PROXY, HTTPS_PROXY, NO_PROXY, etc...) rather having the operator setting proxy via proxy_url.

See https://prometheus.io/docs/alerting/0.26/configuration/#http_config

2 things are happening here:

• Replace proxy_url in favor of proxy_from_environment
• Move the SlackApiToken condition so proxy_from_environment is not dependent on it

[QuentinBisson]

Do we need to do this as it's going away with vintage?

[TheoBrigitte]

Do we need to do this as it's going away with vintage?

Not really, but it's good to align configs between the new and the old Alertmanager until the migration is done.

[QuentinBisson]

Please dont release it before next year :)