decide what to do with `rpmostree` kola tests

In https://github.com/coreos/coreos-assembler/pull/2549, the rpmostree.install-uninstall test was changed to use a local RPM and the discussion around the change raised the question of whether these kinds of tests are actually useful. (To be fair, the discussion of re-implementing some of the rpm-ostree tests in kola had happened earlier as well https://github.com/coreos/coreos-assembler/issues/1176)

The options presented specifically for the rpm-ostree tests:

1. remove them entirely (we have coverage in rpm-ostree upstream)
2. convert them to an external test
3. pick yet another package to use from the CentOS Stream repos

I think I am leaning toward option 1 (even though I had implemented them initially).

(This gets into the larger discussion of how to run upstream tests as part of FCOS, which we might be in a better place to discuss again - https://github.com/coreos/coreos-assembler/issues/1159#issuecomment-587494546)

Start by reviewing pull request 2549, the rpmostree.install-uninstall test, and the earlier discussion in issue 1176. Compare the three proposed approaches with the existing upstream coverage; done means choosing and documenting whether to remove, externalize, or replace these tests.