phosphor-host-ipmid sensor SDRs on dbus

[Patrick Venture]

I sent out an email indicating that we need to make sensors somewhat
dynamic in phosphor-host-ipmid, and either using json or dbus was a
reasonable approach.  I think json would be easier, but, for those
using entity-manager, perhaps having the SDR information supported in
some way associated with the dbus output might be useful.

Idea: Given a sensor, read the SDR from dbus.
Problem: association of a sensor id with a sensor on dbus -- maybe
some search step is run once that caches everything and listens for
change (similar to what James did with phosphor-pid-control)...

I'm just throwing out ideas without full design changes because I want
to get a sense of where the preferences lie before really digging into
what changes are required.

It would support both types (like dbus configuration in phosphor-pid-control).

Patrick