Disable SELinux for live image build test
    
    Building images with SELinux in permissive mode worked fine in
    F29 and F30, but it seems to be broken in F31. Releng seems to
    have it set up so the build environment has SELinux disabled,
    so let's do the same. Will file a bug for this, as bcl says it
    should be considered a bug.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>