Convert --setattr values for attributes marked no_update
    
    Attribute Patrams marked no_update never get cloned to Update commands,
    and thus never receive the `attribute` flag. This makes their `encode`
    method a no-op, which meant they don't get properly encoded when used
    with --setattr, making the --setattr fail.
    
    Introduce a `force` argument to encode, which overrides checking
    for the attribute flag. Use this in set/add/delattr normalization,
    where we know we are dealing with attributes.
    
    https://fedorahosted.org/freeipa/ticket/2616