Add a nodelist option to the slurm launcher

Allow a nodelist to be specified for the slurm launcher. This can either be set
by passing the executable a --nodelist or by setting the env var
CHPL_LAUNCHER_NODELIST
Fix typo
Fix typo
Fix typo
Merge pull request #916 from ronawho/slurm-add-nodelist

Add a nodelist option to the slurm launcher

[reviewed by @thomasvandoren]

Allow a nodelist to be specified for the slurm launcher. This can either be set
by passing the executable a --nodelist or by setting the env var
CHPL_LAUNCHER_NODELIST