LU-6135 lustreapi: allow specific-OST with llapi_layout

LU-6135 lustreapi: allow specific-OST with llapi_layout

Add support to llapi_layout_ost_index_set() for specifying
specific OST indices for a newly created files. Using
ost_index=LLAPI_LAYOUT_DEFAULT to stripe 0 will reset the
specified stripes back to the default.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Ic3aea69527119ddf1cb4f91bac531b6eaa3ebbe5
Reviewed-on: http://review.whamcloud.com/12663
Tested-by: Jenkins
Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>