sudorule: Allow unsetting sudoorder
    
    After setting sudoorder, you are unable to unset it, since the
    check for uniqueness of order of sudorules is applied incorrectly.
    
    Fix the behaviour and cover it in the test suite.
    
    https://fedorahosted.org/freeipa/ticket/4360
    
    Reviewed-By: Martin Kosek <mkosek@redhat.com>