pkg/rhcos/ami: Start supporting RHCOS channels

[wking]

openshift/os@76b3ae5d (openshift/os#314) started adding rhcos_tag=alpha tags to tested AMIs. The likely plan for RHCOS tagging is here, although they don't have stable docs for that yet. With this commit, we allow the user to specify whichever channel they want, and we search for that tag. If they give us nonsense, we'll now hit AWS, see no images in the nonsense channel, and report that back to the caller.

Users that don't care about release channels and just want to use the latest image regardless of channel, can set channel to an empty string.

/hold

We don't want to merge this until we sort out why CI seems to lack alpha tags.

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[cgwalters]

Hmm, the intention was more that the official API was our "AMI JSON" in http://aos-ostree.rhev-ci-vms.eng.rdu2.redhat.com/rhcos/images/aws-us-east-1-tested.json (though note there's a new version of that that just landed recently in http://aos-ostree.rhev-ci-vms.eng.rdu2.redhat.com/rhcos/images/aws-tested.json )

[wking]

Hmm, the intention was more that the official API was our "AMI JSON"...

There was some discussion on this in #forum-coreos on 2018-09-25. My current understanding is that there will be internal RHCOS releases at a faster rate than external RHCOS releases, and that you'd need something like this AWS direct lookup to fine the newest RHCOS release in a given channel that was available to you. Also discussed then was just pushing newest RHCOS releases out publically, which would make sharing a central AMI JSON file between all users possible. I'm fine going either way, but I prefer AWS lookups as long as there is a possibility of different public/internal release cadences.

Also, the public URI is https://s3.amazonaws.com/rhcos-public/aws-tested.json since openshift/os#313 (or at least that PR was part of creating the public bucket). We should probably prefer that.

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@wking: PR needs rebase.

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[crawford]

@wking is this still in progress or has it stalled out? Can we close this if it isn't actionable at the moment?

[wking]

From this and later comments, we're going to stick to a single RHCOS channel for now (although for internal accounts the stream may differ from the public AMIs). We'll revisit once we get multiple channels somewhere. So we may want to restore a way to pin to a specific AMI, but we won't need the AWS tag filtering I'm using here.