Config file environment expansion

[dcseifert]

PR Description

It would be a great feature to expand the configuration file with environment variables like described in issue #280.

Which issue(s) this PR fixes

Fixes #280 Expand config file with environment variables

Notes to the Reviewer

PR Checklist

• CHANGELOG updated
• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[rfratto]

Thanks for working on this!

Following up from our discussion in #280:

Would disabling this feature by default (to avoid unexpected behavior) and enabling it via a switch be a valid option here?

I'd prefer it to work this way. The Loki implementation of this (including the use of github.com/drone/envsubst) would be preferable. Can you make this behavior optional and turn it off by default? Using a flag to enable it should be more than enough.

One other thing: please update the CHANGELOG and give yourself credit for working on this :)

[dcseifert]

Implementation is done and CHANGELOG is updated :)
I tried to stick to the Loki implementation as close as possible.

Looking forward to your review.

[rfratto]

One small nit but this looks great! Do you mind if I add a commit for documenting this in the configuration reference? You're also free to take a crack at it if you want.

[dcseifert]

No, feel free to add the docs!
I just skipped this step for now because I don't want to put the snippet from Loki into a wrong spot.

[rfratto]

Looks like the permission for me to add commits is disabled. I'll just merge this now and follow up with docs. Thanks again!