config: consider sourcing options from files

[lucab]

Afterburn is currently a one-short binary, entirely configured via command-line flags. This is on purpose to keep the complexity (both internally and externally) low.

Our service units are generally written to be easy to override via drop-ins, such as:

[Service]
Environment=AFTERBURN_OPT_PROVIDER="--provider foo"

However, if we start increasing the design complexity (e.g. with a daemon mode, #228), we should consider sourcing file-based configurations.

This would be ideally done via overlayed directories with dropins, plus a simple and standard format (e.g. TOML).

[lucab]

This follows up to @cgwalters's suggestion from openshift/machine-config-operator#964 (comment).

I'm not keen to go down this route right now, but this is pretty much a pre-requisite if we are doing big architectural changes like #228.

[prestist]

Currently we do not have a need for a config file for afterburn. We can revisit this if needed.