`one-sensor-per-pod`:

  Uses `st2.packs.sensors`, defaults come from `st2sensorcontainer`.
  Require `ref` for every sensor in `st2.packs.sensors`.

`all-sensors-in-one-pod`:

  Only use `st2sensorcontainer` for the only pod.
  Ignore `st2.packs.sensors` in this mode.
  No more magic first element in `st2.packs.sensors` to enable this.

This makes it safer to configure one-sensor-per-pod mode, because it
will error if `ref` is not defined when it should be. This way people
cannot accidentally create multiple pods in `all-sensors-in-one-pod` mode.