Add a new stage `org.osbuild.yum.repos` for creating YUM / DNF `.repo`
files in `/etc/yum.repos.d`. All repo-specific options are supported but
only a subset of options which can be set for a repo as well as in the
[main] section are supported.
Add unit test for the new stage.
Fix#907
Signed-off-by: Tomas Hozza <thozza@redhat.com>