LU-276 increase file count to mitigate affect of random condition

LU-276 increase file count to mitigate affect of random condition

in test_18 of ost_pools, increase the count of created file to
mitigate the affect of various random condition which could
lead to false result

Change-Id: Ieca04b37ccc66698cd2159fa1a83f2f9fe07dc41
Signed-off-by: HongChao Zhang <hongchao.zhang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/550
Tested-by: Hudson
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>