schema: properly fix Flag arguments on the client
    
    The previous fix in commit a77e21cbca05be422fe5826857cfba7e0ba6e71f made
    some Bool arguments appear as Flag on the client. This change fixes that.
    
    https://fedorahosted.org/freeipa/ticket/6009
    
    Reviewed-By: David Kupka <dkupka@redhat.com>