Updates for conduit to get MPI working

[vsoch]

Currently, our MPI build will fail because we cannot find MPI and the reason is because we don't have flags set correctly.
From Cyrus:

For MPI, the best path is to set cmake opts:

-DMPI_C_COMPILER=zzz
-DMPI_CXX_COMPILER=zzz
-DMPI_Fortran_COMPILER=zzz

to point to the MPI compiler wrappers (from mpich, etc)

The other case (usually on cray systems) is when you use compiler wrappers as the main compiler.
You pass the cray or mpi compiler wrapper to cmake as CMAKE_C_COMPILER (etc) instead of MPI_C_COMPILER (etc).

In this case you want ENABLE_FIND_MPI to be off. That tells cmake to "trust me, mpi exists but don't look for it!"

See LLNL/conduit#1041 (comment)