The code to search for install tree and repo for image-build and
live-media was only looking at top-level variants. Addons, optional or
integrated layered products could not have been found. This would lead
to error messages such as "There is no variant Server-optional to get
repo from when building live image for Client" even though the variant
exists.
Various tests are updated to exercise this edge case.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>