v2.0.0: fix "MPI processes are invisible to resource managers" issue #1419
Description
Per discussions in Dallas last week, since MPI processes get put into their own process group, they become "invisible" to resource managers. This can lead to orphaned MPI processes left spinning on machines even after resource managers think they have killed the jobs.
Filing this ticket so that we don't forget about it for the v2.0.0 release.