LU-4072 tests: Decrease load on MDT for ZFS in sanity/24v

Due to performance of ZFS, reduce the number of file creates until
LU-2887/LU-4072 are resolved.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I15bc5b69d8ff114bf3cd7fa874c238ecbe29f6f9
Reviewed-on: http://review.whamcloud.com/7870
Tested-by: Hudson
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>