image-build/osbuild: Pull ISOs into the compose
OSBuild tasks can produce ISO files. If they do, we should include them
in the compose, and we should pull them into the iso/ subdirectory
together with other ISOs.
Fixes: https://pagure.io/pungi/issue/1657
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>