avoid delay on commit for PMI task at rank 0

Intel MPI, on MPI jobs initialisation through PMI, uses to call PMI_KVS_Put()
many many times from task at rank 0, and each on these call is followed by
PMI_KVS_Commit(). Slurm implementation of PMI_KVS_Commit() imposes a delay
to avoid DDOS on original srun. This delay is proportional to the total number.
It could be up to 3 secs for large jobs for ex. with 7168 tasks. Therefore,
when Intel MPI calls PMI_KVS_Commit() 475 times (mesured on a test case) from
task at rank 0, 28 minutes are spent in delay function.
All other tasks in the job are waiting for a PMI_Barrier. Therefore, there is
no risk for a DDOS from this single task 0. The patch alters the delaying time
calculation to make sure task at rank 0 will does not be delayed. All other
tasks are globally spreaded in the same time range as before.