Great question. With PGI 2014, MPICH was updated to version 3.0.4. As of the 14.4 update, we noticed that the “remote execution method” question really applied to the old MPICH1 version, and was functionally obsolete with the newer MPICH. As a result, we removed it as part of a general cleanup of the MPICH install script for 14.4.
MPICH 3.0.4 now dynamically selects a transport mechanism at runtime. You can optionally override this behavior (and force a choice, say rsh or ssh) via the -launcher flag to mpirun/mpiexec. You can also set the $HYDRA_LAUNCHER environment variable to use the desired launcher if you would rather not pass this flag every time.
This page has more information, which you may find helpful:
Hope this helps!