Fix MultiprocessingRunner
Fix MultiprocessingRunner

* Do not re-submit task: "node not in results"

* Second argument to pool.apply_async is "args" so it must be
  a list of arguments.

* Calling submit_ready_tasks in callback does not work.
  Probably AsyncResult is not set to "ready" state yet
  at the time callback is called?

* r.get() instead of r.wait() to raise an error if something went
  wrong in subprocess.

tkf/buildlet