LU-3664 test: get_param atime_diff returns multiple values

LU-3664 test: get_param atime_diff returns multiple values

In sanityn test 23, lctl get_param mdd.*. time_diff can
return multiple values. This can happen with there are multiple
MDTs mounted. In order to deal with multiple values, we run the
return value of lctl get_param through "head -1".

Test-Parameters: mdtcount=2 mdscount=2
Signed-off-by: James Nunez <james.a.nunez@intel.com>
Change-Id: Ie830429af38e1e4bce084aa1e6490bbadf366920
Reviewed-on: http://review.whamcloud.com/7750
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>