task/mpi: Explicit check for version mismatch

Instead of proceeding to have mpiexec fail or hang,
do an up-front check that the MPI version is the same
on all of the nodes in the test.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #300 from ceph/wip-mpi-vers

task/mpi: Explicit check for version mismatch