RegCM recipe does not list mpi as a 'run' dependency

I built RegCM with OpenMPI then loaded RegCM: 

```sh
spack load regcm
```

I then expected `mpirun` to be on my `$PATH` but it wasn't.

I think this is because within [the RegCM recipe](var/spack/repos/builtin/packages/regcm/package.py)) `mpi` is only listed as a `build` and `link` dependency.  

Spack version: 9a7a532084356a473a282b0443da40f05961327f