Fix validator for SELinux user map settings in config plugin.
    
    We need to compare two values and need to be aware of where those
    values are coming from. They may come from options, setattr or
    existing config. The format of that data is going to be different
    depending on its source (always a list internally).
    
    One may also set both at the same time so a standard validator cannot
    be used because it lacks the context of the other value being set.
    
    https://fedorahosted.org/freeipa/ticket/2938
    https://fedorahosted.org/freeipa/ticket/2940