install: fix command line option validation
    
    The code which calls the validators was accidentally removed, re-add it.
    
    https://fedorahosted.org/freeipa/ticket/5386
    https://fedorahosted.org/freeipa/ticket/5391
    https://fedorahosted.org/freeipa/ticket/5392
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>