This machinefile management approach will work in any environment
supporting specification of machinefiles to the mpirun wrapper.

* MPICH, MVAPICH, M*PICH typically support this.

* OpenMPI typically supports this, with at least one known EXCEPTION:
OpenMPI version < 1.3 in a a torque-managed environment or other
situation where the resource manager is tightly integrated with
OpenMPI, orterun will ignore machinefiles or error.
