Fix encoding for setattr/addattr/delattr
    
    Attribute values passed by --{set,add,del}attr parameters were
    normalized and validated using appropriate parameter, but were
    never encoded for the backend. This make prevents manipulation
    with dirsvr BOOL attributes where framework tries to pass
    boolean value instead of encoded "TRUE"/"FALSE" values.
    
    https://fedorahosted.org/freeipa/ticket/2418