Changes

kola/kolaTestIso: use `kola-junit` to publish JUnit test results

This will allow us to much more easily inspect failing kola tests.
Goes together with https://github.com/coreos/coreos-assembler/pull/3667,
which has more details.

While we're here, use `kola-testiso` as marker since it looks clearer
and for consistency with `kola-upgrade`.