- on some arches (also Fedora x86_64) systemd-nspawn may not be
available
- delete composes from other tests in rlPhaseStartCleanup because
we're seeing the tar compose kind of hanging in Jenkins and that
test script is executed last so the slave may be running out of
disk space. Be a good citizen and clean up after the previous
tests.
Related: rhbz#1656105
