backend/director admin states

[Dridi Boukelmoune]

> Best proposal for "???" seems to be "auto", even though it is not
> entirely on point.
>
> The implementation can control its view of the health two ways
>
>   A) Call VRT_SetHealth() and set it on the director.
>      (useful for probes as we know them)
>
>   B) Provide a ->healthy() method which will be asked whenever
>      anybody wants to know.
>      (useful for selecting directors like rr/fb/rnd/hash)

"auto" is what we have today in the CLI, and I don't remember ever
being confused by that when I was only a user. And it seems to
still match your A) and B) cases.

Dridi