Feature request in phosphor-pid-control

[Lei YU]

> So, this change really is just, "don't fail if you can't find the
> sensor during start-up, and keep trying until it shows up"
>
> Does that follow?

Pretty much close.
More like "in any case, don't fail if you can't find the the sensor, and keep
trying".
Note this is applicable to sensors that are unavailable.

For sensors that are expected to be treated as fault, it should trigger
failsafe, and this is discussed in Ed's mail.

Putting it together, the feature request is splitted:
1. For sensors without operation status, we can config them as optional, so
   they do not trigger failsafe;
2. For sensors with operation status, do not trigger failsafe if it is
   unavailable; trigger failsafe if it is fault.