Fix for backfill launch job with reboot

This bug was likely the root cause of bug 3366. If the backfill scheduler
  allocates resources for a batch job and a node reboot is required, the
  batch launch RPC would be sent to the agent. At that point, there is a
  race condition between the agent and the job_time_limit() function
  testing for boot completion. If the job_time_limit() function ran
  first, it would trigger a second launch RPC request getting sent to
  the agent.
bug 3366