Import FCOS trans.py that converts meta.json → release.json

[cgwalters]

This is a straight up import of almost-exactly the contents
of https://github.com/coreos/fedora-coreos-releng-automation/blob/master/coreos-meta-translator/trans.py

Prep for adding more stream tooling into coreos-assembler as
part of openshift/os#477

[cgwalters]

To actually use this for RHCOS we'll need various improvements like not having a hardcoded URL, but I think those changes should be followups.

How hard would it be to change the FCOS pipeline to consume this after it lands? We'd just need to schedule a cosa pod in the pipeline right?

[jlebon]

How hard would it be to change the FCOS pipeline to consume this after it lands? We'd just need to schedule a cosa pod in the pipeline right?

We already do everything in a cosa pod, so it should just be a simple change from /var/tmp/fcos-releng/coreos-meta-translator/trans.py to coreos-assembler generate-release-meta. I can line up that PR.

/lgtm

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[jlebon]

Another obvious follow-up now that it's in cosa is to auto-default to $PWD for --workdir, and latest for --build-id.