LU-3782 test: Fix for faliure when no file are created.

LU-3782 test: Fix for faliure when no file are created.

The test_18 of ost-pools.sh failed when the createmany
function returns zero and test showed a divide by zero
error. Fixed the issue by adding a check for the return
value and also fixed incorrect average calculation.

Seagate-bug-id: MRP-1117
Signed-off-by: Kirtankumar Krishna Shetty <kirtan.shetty@seagate.com>
Change-Id: I13ea0c1aeb974b5e74dfc7d92fe4e3b744a1fba6
Reviewed-on: http://review.whamcloud.com/16939
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>