Spawn and MPI_COMM_SPAWN with the SOFT info

[abouteiller]

It appears that there is no attribute currently defined in the PMIx_spawn operation to pass the behavior defined as a 'SOFT' spawn in MPI.

In an MPI soft spawn, the MPI_COMM_SPAWN operation requests that N new processes be spawned, but it is acceptable that N-x processes are actually created (and this is not an error).