LU-8738 tests: ladvise dontneed test write to single OST

LU-8738 tests: ladvise dontneed test write to single OST

sanity test 255b exercises the ladvise hint 'dontneed' by
checking total cache and cache used on a single OST. Limit
the file striping to a single OST for the file created for
this test.

Test-Parameters: trivial testlist=sanity,sanity,sanity
Test-Parameters: trivial testlist=sanity,sanity,sanity
Test-Parameters: trivial testlist=sanity,sanity,sanity
Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: Iee08576726fc56bc9e7aa961c22819265c31f69b
Reviewed-on: http://review.whamcloud.com/23867
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>