Make core Workstation packages mandatory
    
    This makes core Workstation components, including the 'Core
    Applications' from the technical specification, mandatory in
    comps. This means that if they are removed, the group they are
    in (gnome-desktop or firefox) is no longer considered to be
    installed, and compose of any images containing these groups
    will fail if any of the packages cannot be installed (which is
    the main goal of the change; to avoid the images composing but
    missing these key packages).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>