LU-9140 nrs: measure the runtime of dd directly

This patch changes the way to measure the runtime of "dd". Instead
of parsing the output of "dd", we use date command to calculate
the runtime of dd directly, avoiding the parsing failure caused
by changed output format of "dd".

Change-Id: Ibd2e3963f791404ee927981238227012cf4dbf2c
Test-Parameters: trivial testlist=sanityn
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/27878
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>