Proposal for Reorienting Two-Week Atomic (Ticket #6313)

[Adam Miller]

Hello all,
    This is a proposal for implementing a solution for releng ticket #6313[0]

Currently the compose (pungi) produces an Atomic ostree, installable ISO, and
various cloud images nightly. Those artifacts are then tested via AutoCloud.
I'm proposing that we add a second "new" repo, every two weeks we will sync
the latest commit from the current repo/ref that has passed testing in
AutoCloud then using the corresponding compose, reproduce the ISO and various
cloud images using the exact same content but pointing to the new remote ref
(which we will call fedora-atomic/24/x86_64/stable/docker-host) that is
only updated every two-weeks. This would be added to the current two-week
atomic release script instead of impacting pungi or other current processes.

We then will add documentation for users who want to switch between release
streams via different repos/refs, either nightly updates.

The main concerns are:
- We would need to manage the versioning of the ref that gets synced over
  ourselves but I think we can just version it as the compose id for
  consistency.
- Need to figure out how to extract the ostree repo/ref commit from AutoCloud
  to ensure we're getting the right thing. This should be predictable though
  since the artifacts are built from an ostree created in the compose.

[0] - https://fedorahosted.org/rel-eng/ticket/6313