LU-1962 util: Add --lazy flag to lfs df

LU-1962 util: Add --lazy flag to lfs df

add --lazy flag to lfs df
it allows to skip unavailable OSTs and report
"Resource temporarily unavailable" for them

Xyratex-bug-id: MRP-583
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Alexander Boyko <alexander_boyko@xyratex.com>
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Change-Id: Idb6b728b52c5fa1590d07201700a3fad0ef7cc78
Reviewed-on: http://review.whamcloud.com/4007
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>