LU-2008 utils: quote TUNE2FS options in ldiskfs_label_lustre

LU-2008 utils: quote TUNE2FS options in ldiskfs_label_lustre

This patch improves ldiskfs_label_lustre() to quote the options
for TUNE2FS in case the label contains space(s).

Test-Parameters: envdefinitions=SLOW=yes \
clientcount=4 osscount=2 mdscount=2 \
austeroptions=-R failover=true useiscsi=true \
testlist=mmp

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I3eaeb604517dba29e07f9750aeb3afed9443e94f
Reviewed-on: http://review.whamcloud.com/6077
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>