RFC: config: add `streams[].skip_cloud_images`

[jlebon]

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In RHCOS, we're often in a state where we don't care about regenerating all the disk images and instead just want the containers. We could do this by manually adding all the images we don't care about as skip_artifacts entries, but that's cumbersome and error-prone.

Add a skip_cloud_images knob for this. We still generate the QEMU and live artifacts for kola tests to run and because they're useful to debug, but we drop everything else.

[jlebon]

@mike-nguyen brought up that if we do this, it breaks the current flow of retroactively selecting cloud images from an already released z-stream when preparing a bootimage bump and replicating that. So either we'd need to change that process so that we turn it back on in time for the z-stream release when we know bootimage bumps are coming, or e.g. we add a new job (or enhance cloud-replicate) to "complete the build" with all the cloud images. The complexity there doesn't seem worth it.