Add RHELCoreOSExtensions with Azure image URL

[cgwalters]

This contains just a URL for now because that's all
the current RHCOS cosa metadata has. I'm trying
to add e.g. the Azure Blob storage md5 information as
well as the full size+sha256, but in practice this
data is just the uncompressed VHD; anyone who wants
to do "offline" verification outside of Azure can
replicate that.

For FCOS we may end up uploading the image too,
though we hope there to end up in the Marketplace.

For now, let's stick this off an explicit extension area.

Closes: #13

[cgwalters]

OK lifting draft, I've tested this in concert with coreos/coreos-assembler#2026

[ashcrow]

The code looks good to me. If RHCOS stream info is also meant to be supported going forward I recommend eventually updating the README.md as well.

[jlebon]

Remind me again, is the stream metadata for RHCOS going to be a public API, or just for us (where "us" includes oc, openshift-install, operators, etc...) ?

[cgwalters]

In order to support e.g. AWS UPI installs, it basically has to be a public API.

I mean, the way it works now is that:

• CoreOS team bumps pinned installer
• docs team later comes along and copies those AMIs into docs
• Some human doing a UPI (hopefully with custom scripting) pastes the AMIs into their tools

Which is obviously, horrendously broken.

[jlebon]

One comment, but seems reasonable to me.

[bgilbert]

I'm okay with the general approach. If we could publish an FCOS VHD to a storage container before we had proper Marketplace support, I'd lean more toward #14, but I think blob uploads are also blocked on coreos/fedora-coreos-tracker#148. I like separating the RHCOS hacks off into an omitempty RHCOS-hack-area; hopefully that'll make them easier to clean up later.

[cgwalters]

OK, addressed comments!