Run a basic cluster test in Prow

[jlebon]

This is somewhat part of openshift/enhancements#1637, but we should track it separately.

One obvious omission in this repo is any cluster tests in CI. As we recently learned, there is currently no testing that happens downstream between the production pipeline build and ART pushing its output to nightlies. This is something that should be fixed, but additionally we really should have CI in place here.

This has historically been tricky to do because, well, RHCOS is just built differently from how other components are built. Now that we're moving to layering, this should become easier. Eventually, this repo will be purely about the contents of the OCP node layer and it'll be built as a layered image just like any other OCP component.

Obviously we'll need to be able to test new RHEL content too with OpenShift, and that could happen here (e.g. if we have base image digest bump PRs) or higher up where RHCOS definitions would now live as part of reverse dependency testing (though obviously, if it lives in Gitlab, that's much harder to pull off with Prow which is very Github-centric).

Anyway, in the short-term, it should be possible to use some of the layering trickery to get the oscontainer built by cosa build ostree as an output of a container build, which then can be fed to a cluster upgrade test (e2e-aws-ovn-upgrade was mentioned as a good target).