Prevent vtkProcessModule from calling MPI_Finalize() twice

This sets the FinalizeMPI flag in vtkProcessModule after calling
MPI_Finalize() in order to prevent the class from calling MPI_Finalize()
twice.

Change-Id: I38b46250e1df27e45d83c5ea256ecfccfe4fa731
Merge topic 'fix-mpi-finalize' into next

3a80e90b Prevent vtkProcessModule from calling MPI_Finalize() twice