Once again, the Slurm folks have decided to redefine their envars, reversing what they had previously told us to do. So cleanup the Slurm allocation code, and also adjust to a change in srun behavior that now aborts a job if the ntasks-per-node doesn't get specified when ORTE calls it, but the user specified it when getting an allocation. Sigh.

cmr=v1.7.4:reviewer=miked:subject=Update Slurm allocation and launch