ostree-installer: Only run on empty variants
    
    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>