Use config file for gitlab-k8s-agent

What does this Merge Request do?

Relates to gitlab-org/cluster-integration/gitlab-agent#16 (closed).

Going forward, gitlab-k8s-agent (kas) will mainly be configured using the config file.

In this MR, we move away from flags to an auto-generated configuration file. This will also removes friction from developing features that require new configuration, as currently one has to modify the Procfile to add this flag. This way, one only has to modify the configuration file, which is auto-generated with working defaults.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Documentation added/updated, if needed.
• gdk doctor test added, if needed.