Allow use of system MPI.

The pre-existing cmake script magic for falling through
to calling FindPackage(MPI) to get the system MPI works fine,
as long as we use the name MPI instead of OPENMPI, which
is just the variant of MPI we happen to build.

NOTE: You must set CDAT_BUILD_ESMF_COMM accordingly as we have
no introspection that will detect the particulat flavor you have.
Merge remote-tracking branch 'remotes/devel/allow_system_mpi'

* remotes/devel/allow_system_mpi:
  Allow use of system MPI.