Fix for node state when shrinking jobs

When nodes have been allocated to a job and then released by the
  job while resizing, this patch prevents the nodes from continuing
  to appear allocated and unavailable to other jobs. Requires
  exclusive node allocation to trigger. This prevents the previously
  reported failure, but a proper fix will be quite complex and
  delayed to the next major release of Slurm (v 16.05).
bug 1851