Tracker for cleanups possible after RHCOS moves to FCOS pipeline

[jlebon]

This is a tracker for all the cleanups we should be able to do once we've fully moved over RHCOS to the new pipeline.

• delete cosa push-container; we only use cosa push-container-manifest now for all our containers
• delete upload-ami; we use pure cosa buildextend-aws --upload now with support for granting to additional users, scheduling AWS tests and marking the AMIs public: scripts: drop upload-ami openshift/os#1074
• delete cosa buildextend-extensions; this was the legacy way to publish extensions
• make digest required again in images (see cmd-push-container-manifest: change image key schema #3129) (though this is more dependent on branching on 4.12)
• delete cosa build-extensions-container backcompat alias
• delete cosa buildextend-legacy-oscontainer; we don't need to build or ship the legacy oscontainer anymore

[dustymabe]

@jlebon bump

The RHCOS and FCOS share the same pipeline code now.

[jlebon]

I think we're still dependent on the legacy oscontainer for now. https://issues.redhat.com/browse/MCO-392 tracks its removal.