Update mpi example to use hostfile

**Is this a BUG REPORT or FEATURE REQUEST?**:

/kind bug

**Description**:

It's better to use `--hostfile` for mpi job instead of parsing it to string:

```shell
% mpirun [ -np X ] [ --hostfile <filename> ] <program>
```

xref https://www.open-mpi.org/doc/v3.1/man1/mpirun.1.php