LU-2887 tests: Reduce performance expectatiomn for zfs

LU-2887 tests: Reduce performance expectatiomn for zfs

Drop ZFS Performance expectation to below observed values.  This is a
temporary measure until ZFS performance issues are dealt with.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I86c025e15ba67e86269344faa2ea56be0966a873
Reviewed-on: http://review.whamcloud.com/7848
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>