This adds a function, CleanupOldCacheDirs, that checks the dirs under
/var/cache/osbuild-composer/rpmmd/ and removes files and directories
that don't match the current list of supported distros.
This will clean up the cache from old releases as the are retired, and
will also cleanup the old top level cache directory structure after an
upgrade.
NOTE: This function does not return errors, any real problems it
encounters will also be caught by the cache initialization code and
handled there.
