config: add streams[].skip_disk_images

As part of openshift/enhancements#1637, we've
moved OCP 4.19 to use bootimages with RHEL content only. This means that
the bootimages built with OCP content will never be used in practice.

Add a `skip_disk_images` knob for this.

We still generate the QEMU image for kola tests to run and because
they're useful to debug, but we drop everything else. (Actually, we
could also not generate the QEMU image either and sanity-check the OCI
image with `kola run --oscontainer`, but that requires more rewiring.)

We don't generate live media since I don't think the test coverage from
that is meaningfully different enough from the RHEL-only variants given
that the additional OCP packages are unrelated.

docs/config.yaml

@@ -59,6 +59,10 @@ streams:
       variant: rhcos-9.0
       # OPTIONAL: override whether to use a versionary for this stream
       versionary: false
+      # OPTIONAL: Whether to skip disk image generation and upload. This
+      # is equivalent to listing all live artifacts and cloud platforms in
+      # `skip_artifacts`.
+      skip_disk_images: true
     stable:
       type: production
       # OPTIONAL: override cosa image to use for this stream

utils.groovy

@@ -425,6 +425,11 @@ def get_artifacts_to_build(pipecfg, stream, basearch) {
         artifacts -= pipecfg.streams[stream].skip_artifacts?.all ?: []
         artifacts -= pipecfg.streams[stream].skip_artifacts?."${basearch}" ?: []
     }
+    if (pipecfg.streams[stream].skip_disk_images) {
+        // Only keep the extensions container. Note that the ostree container
+        // and QEMU image are always built and not skippable artifacts.
+        artifacts = artifacts.intersect(["extensions-container"])
+    }
     return artifacts.unique()
 }