Add --spread-job option

Add salloc/sbatch/srun --spread-job to distribute tasks over as many nodes
    as possible. This also treats the --ntasks-node-node option as a maximum
    value. For example "srun -n16 --ntasks-per-node=2 --spread-job ..." will
    run the job on at least 8 nodes (with 2 tasks each), but will attempt to
    use as many nodes as possible up to the task count (1 task per node).
bug 2732