shutil.rmtree should be avoided in almost all cases
safe_rmtree has its usage in tasks module, but innards are replaced with
koji.util.rmtree, so we don't have two implementations of same task
Related: https://pagure.io/koji/issue/648
This is a different take at https://pagure.io/koji/pull-request/45
Instead of installing just the runroot builder plugin explicitly, here we
separate the plugins out into hub and builder plugins explicitly and install
each type in turn.
