This changes the default to 0, to avoid yields during progress in srun.
In mpirun, ompi_mpi_yield_when_idle is set to 1 if oversubscribed
otherwise 0. But the default is 1 though, and it is used in srun.
Now srun and mpirun have the same latency in non-oversubscribed cases.
Signed-off-by: Piotr Lesnicki <piotr.lesnicki@atos.net>