dispatch: Fix rules for running the partitioning step.
    
    This step is not scheduled by default, it is skipped for any text
    installation and many kickstart commands (like autopart), it is requested
    if the user selects "review partitioning layout" on the parttype screen.
    
    There is a mechanism in autopartitioning that enables manual partitioning
    screen (if available) when autopartitioning fails. Instead of checking the
    availability this just tries to request the manual partitioning step and
    catches DispatcherError for those cases where this is not possible.