Set correct productmd image type for osbuild installer image
    
    In the productmd metadata, these osbuild-built installer images
    have their 'type' as "dvd". This is not correct: the "dvd" type
    is meant to be a traditional RPM-based installer image, like the
    old Everything DVD or the current Server DVD. The correct "type"
    for an ostree installer image is "dvd-ostree". (There is also,
    now, "dvd-ostree-osbuild", added in
    https://github.com/release-engineering/productmd/pull/170 ,
    which is intended for the case where the osbuild image is an
    experimental one, and there is still an image built with the
    older system that is the 'official' one.)
    
    This 'manifest_type' thing was added after the discussion in
    https://pagure.io/pungi-fedora/pull-request/1190 , to let us
    specify the productmd type for osbuild-built images correctly.
    In this case we don't want to use "dvd-ostree-osbuild" because
    from F40 onwards it seems we want the osbuild-built image to be
    the official/main one, so it should have the "official" type.
    
    The practical consequence of this at present is the F40 IoT
    composes aren't getting openQA testing, because openQA relies
    on this property to identify the image, and it's expecting to
    see "dvd-ostree".
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>