when testing downstream snapshots this makes sure that
lorax-composer and composer-cli are coming from the host OS.
We also make 100% sure that there are no other repositories inside
the VM other than what has been configured on the host!
Make it possible to override where repo files are copied from by
defining the REPOS_DIR variable. By default the value is
/etc/yum.repos.d
Cherry-picked from bd2f4925de
Related: rhbz#1769525
to help with running the tests by hand on downstream snapshots.
In that scenario we want TEST_OS/VM_IMAGE to look as closely as
possibly like the snapshot that we'd like to test.
Cherry-picked from 1351c4dc63
Related: rhbz#1769525
