LU-3244 utils: tunefs.lustre should preserve virgin label

so that the filesystem registers can register properly on MGS,
if tunefs.lustre was used right after mkfs.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I6245e5e4d10cd0a13a4e9068a9b758da8580b537
Reviewed-on: http://review.whamcloud.com/6216
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>