LU-6385 tests: Sync OSTs between tests in obdfilter-survey

osd-ldiskfs and osd-zfs operate differently with regards to writeback
cache.  This adds a force sync between test runs so that a test list
of "write read" will have all writes finished before reads starts for
both ldiskfs and zfs.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I3414a595220d9eb65145f8246ed5e243c112b866
Reviewed-on: http://review.whamcloud.com/14143
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>