LU-4108 tests: decrease load for zfs in performance-sanity/4

LU-4108 tests: decrease load for zfs in performance-sanity/4

Given the poor performance of metadata on ZFS (cf LU-2600), this patch
decreases the number of files created (similar to what happens in
perforamnce-scale.sh).

Test-Parameters: mdtfilesystemtype=zfs ostfilesystemtype=zfs  mdsfilesystemtype=zfs testlist=performance-sanity
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I998c8525bf218dacad07d98ac842f4c12102cd9a
Reviewed-on: http://review.whamcloud.com/9725
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>