LU-4748 test: get one single qos_threshold_rr number

LU-4748 test: get one single qos_threshold_rr number

In sanity.sh test_116b, if there are multiple MDTs on the same MDS,
we should get one single qos_threshold_rr number instead of the array,
otherwise it will cause resetting qos_threshold_rr failure.

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: Ie779769f7ee35cd118fb6412fe3352a03254b9d3
Reviewed-on: http://review.whamcloud.com/9580
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>