Prevent changing job HOLD reason set by select plugin

Under some conditions the select/cons_res plugin will hold a job,
  setting it's priority to zero and reason to HELD. The logic in
  slurmctld's main scheduling loop previously kept its priority
  at zero, but changed the reason from HELD to RESOURCES. This
  change leaves the proper job state as set by the select plugin.
This may be related to bug 1750