launch/poe - fix network value

if an job step's network value is set by poe, either by directly
executing poe or srun launching poe, that value was not being
propagated to the job step creation RPC and the network was not
being set up for the proper protocol (e.g. mpi, lapi, pami, etc.).
The previous logic would only work if the srun execute line
explicitly set the protocol using the --network option.