If the variant is not empty, it buildinstall will run lorax on it and
create some files that would otherwise be overwritten by the ostree
installer.
The validation script is updated to load variants file as it needs a
list of variants to find out if the config is wrong.
Fixes: https://pagure.io/pungi/issue/695
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>