LU-7428 test: commit the label change to disk

LU-7428 test: commit the label change to disk

when mounting some Lustre target (MDT or OST), the device label
will be changed if it is mounted for the first time, and it's
better to commit the change to disk.

Change-Id: I36125356bf19f0aedcb1a6939e7461911c1a749d
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/18871
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>