kola/distros: introduce a rhcos-base variant

[jbtrystram]

As we introduced pure rhel and rhcos variants in [1], we did not change the name key in meta.json

Kola uses this key to determine the distribution of the image to discriminate relevant tests.
Some tests are tied to OCP content, e.g. crio.base and crio.network [2] and inherently won't work
with the base because they rely on content that
is added as part of the node image layer.

Currently we skip the tests tagged with openshift in prow [3] and pipelie to work around this.

Going with the principle of least disruption
by adding rhcos-base rather than adding rhcos-ocp because all of the build already out there contains the old name and a I don't want to break compatibility if someone use a newer cosa to test those older
builds.

[1] openshift/os#1445
[2] https://github.com/coreos/coreos-assembler/blob/main/mantle/kola/tests/crio/crio.go
[3] https://github.com/openshift/os/blob/48a18918794f5418352c03a3415fac3fde28e1b6/ci/prow-entrypoint.sh#L306

See openshift/os#1790

[jbtrystram]

I did this then realized that we simply do --tag !openshift, but I thought this was still worth to open the PR to discuss this.
I still think this make more sense in the long run

[dustymabe]

yeah. --tag !openshift is there.

I'm not sure if this is the change we'd want to make specifically. Will see what others think.

[jlebon]

We discussed this OOB. But just to summarize I think instead what we'll want to do is once we've switched the default variant from ocp-rhel-9.6 to rhel-9.6, we can flip the openshift tags to be required tags. That'll make them not run by default so that you don't have to specify --tag !openshift at all.