scheduler: disable broken heartbeat mechanism
scheduler: disable broken heartbeat mechanism

Problems:

- puts offline/retired boards back online
- the heartbeat part should run on the worker shedulers, while the part
  that updates the status based on the last heartbeat should only run on
  the master scheduler!
- the state transition is broken:
  - lets say there was a job running.
  - heartbeat fails, device status goes to UNREACHABLE
  - job still shows running
  - when the heart beat passes againg, the device will go to IDLE
  - another job queues
  - and we have two jobs running

Change-Id: If6ede3e0f506bad9c81aa82d72a1a0e245f1417e
Reviewed-on: https://staging.review.linaro.org/495
Reviewed-by: Tyler Baker <b3cc311ecf91e7c5c184946246e38d36003af74d@linaro.org>

Linaro/lava-server,Linaro/lava-server,OSSystems/lava-server,OSSystems/lava-server,Linaro/lava-server,Linaro/lava-server,OSSystems/lava-server