Deprecating SLURM_JOB_NODELIST in chpl_launchcmd

This is no longer a valid slurm env var, instead we now pass the nodelist
directly to our launcher by passing down CHPL_LAUNCHCMD_HOSTLIST with the newly
added --nodelist flag
Merge pull request #917 from ronawho/get-correct-xc-nodes

Deprecating SLURM_JOB_NODELIST in chpl_launchcmd

[reviewed by @thomasvandoren]

This is no longer a valid slurm env var, instead we now pass the nodelist
directly to our launcher by passing down CHPL_LAUNCHCMD_HOSTLIST with the newly
added --nodelist flag