Preemption logic could hold job

If a job can only be started by preempting other jobs, the old logic
  could report the error:
  "cons_res: sync loop not progressing, holding job #"
  due to the usable CPUs and GRES needed by the pending job not
  matching. This change prevents the error message and job hold
  when job preemption logic is being used. The error message and
  job hold still take place for job scheduling outside of preemption,
  which will match CPUs and GRES at the beginning.
bug 1750