Build bootc images in IoT variant, tweak subvariants and names
    
    There's substantial history here, including
    https://pagure.io/fedora-infra/ansible/pull-request/2295 and
    the discussion in the PR for this commit -
    https://pagure.io/fedora-iot/pungi-iot/pull-request/102 . This
    is trying to make the metadata for these images accurate and
    informative.
    
    With this change, the variant for both images will be "IoT" and
    they will both appear in the IoT directory in the tree. The IoT
    image's subvariant will also be "IoT". The base image's
    subvariant will be "base". The format for both images will be
    "ociarchive", as it currently is. From tomorrow, the type for
    both images will be "bootable-container". The filename for the
    IoT image will start with "Fedora-IoT-bootc", and the filename
    for the base image will start with "Fedora-base-bootc". So we
    will have these basename - subvariant - type - formats:
    
    * Fedora-IoT-bootc - IoT - bootable-container - ociarchive
    * Fedora-base-bootc - base - bootable-container - ociarchive
    
    The base image can easily be moved into the main compose and
    maintain the same filename base, subvariant, type and format. It
    would have a different variant in that case, but in practice we
    do not use the variant for image identification purposes much
    anyway.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>