Build bootc images in IoT variant, tweak IoT image's subvariant
    
    As discussed in
    https://pagure.io/fedora-infra/ansible/pull-request/2295 , this
    subvariant is more usable and in line with previous practice. It
    will make life a bit easier for fedora-image-uploader. This also
    tweaks the config to build both ostree container images in the
    IoT variant and give them explicit subvariants, instead of
    building them in their own variants and relying on the variant
    to be inherited as the subvariant definition too. This just
    seems a bit cleaner and avoids these variants showing up in the
    metadata, which reads a bit oddly.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>