Update launchcmd-for-aprun-launcher to accept optional hostlist argument.

The hostlist will be passed to qsub to specify certain nodes to reserve. See
moab documentation for more detail:

  http://docs.adaptivecomputing.com/maui/7.1.5managingreservations.php

r: @eronaghan

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23660 3a8e244f-b0f2-452b-bcba-4c88e055c3ca