Add scripts to build and run MPI tests with MVAPICH2-GDR

[kingchc]

Two new scripts are added to build and run COMB (MPI datatype tests) with MVAPICH2-GDR. Tested on POWER9 systems like LLNL Lassen.

Build:

./scripts/lc-builds/blueos/mvapich2-gdr_nvcc_9_2_gcc_7_3_1.sh

Run:

./scripts/run_tests_mvapich2.sh

NOTE: These scripts can be modified manually to test different versions of MVAPICH2 (or any MPI library).

Any suggestion will be appreciated.