fix hierarchical mns w.r.t. MPI installed with a dummy toolchain

enhanced version of #983 by @kcgthb (which should have been opened targeting `develop`
I pinged @rtmclay about this. At TACC, they treat Intel MPI as a compiler-dependent module.

In other words, they basically install Intel MPI with a particular (non-dummy) compiler toolchain (e.g. `iccifort` or `GCC`).

That's the best solution I can think of. Applying this across the board (for all existing toolchains involving impi) requires to create new impi easyconfig files though, which use non-dummy toolchains...

I don't think that a **big** issue, but it's painful.