Make all configuration items configurable with environment variables

[sando38]

Currently, only some configuration items like secret can be configured using environment variables.

Goal:
Implementing a general procedure e.g. with a prefix ETURNAL__ to let eturnal evaluate these variables for its configuration items.

For complex configuration items like listeners, something like this could be used: ETURNAL__LISTEN_0_PORT=443

[sbe-arg]

This would be amazing so docker does not rely on the config yaml file.

[JoelJoos]

+1