[NRRPLT-4809] Use mvapich2 mpirun syntax.

We are transitioning from OpenMPI to mvapich2, which has different
syntax for mpirun. This minor change updates the syntax and will ensure
that both local and deployed installed are using mvapich2.

The mpirun (mpiexec.hydra) is now a bit different than the OpenMPI
behavior. When executed, it sends a SIGSTOP to the parent process tree
and suspends everything (the bash terminal and our Python processes),
so it's necessary to now spawn it using the os.setssid parameter to
start a new session for the subprorcess that is separated from the
main process tree.

This patch also removes the subprocess.wait() call which according
to the documentation can cause buffering/memory issues. In it's place
poll() is used with live/faster output of the processes output to the
main log.

Change-Id: I3a9e255986ac6f39a01b7a1a801e8ab24a14abac