POC for gathering upstream tests into a container

We landed support for "exttests" in coreos-assembler: https://github.com/coreos/coreos-assembler/blob/98d40e6bb13adc02bcd5f02f1d5bff7bafa0780d/mantle/kola/README-kola-ext.md

Since then we are successfully running upstream test suites using kola on upstream pull requests.

But we also want the inverse: run upstream tests on builds outside of PRs to those repos.

For example, I really want to run ostree's "transactionality" test on FCOS builds so that if a kernel change breaks it, we know that.

Proposal

• Build this container in api.ci like we're building the cosa buildroot; currently done at registry.svc.ci.openshift.org/cgwalters/cosa-exttests:latest
• Change the FCOS/RHCOS pipelines to pull this container and do: kola run ... ext.* (This raises some issues like the fact that we'd need to store/share the images in a way that would allow a secondary container to access them)

Alternative: coreos-assembler only

Fold this into coreos-assembler.

Alternative: Make RPMs of tests and install those in coreos-assembler

We have an ostree-tests RPM...but I'm not sure I want to deal with packaging for it.