Fix discovery of existing raid/lvm for ks install without clearpart (#503310) (#503681)
    
    Default clearPartType to None so that all devices are discovered during storage
    initialization step, and UI parttype can be set to its default "Replace existing
    linux system".  Also set clearPartType appropriately when going back from partition dialog
    so that all devices are discovered during storage reset.
    
    There is one case that can't be resolved this way: clearpart --all or --linux
    is set in ks and "Create custom layout" is selected in parttype UI step. Because of ks
    setting, storage initialization doesn't discover devices (RAID, LVM) on
    partitions that are to be cleared and therefore they are not present in custom
    partition dialog. This can be workarounded by going back and than again to
    custom partitioning dialog. I think we should grey-out type of partitioning
    combo set to value from ks clearpart command.