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.

This is a backport of
Lustre-commit: b4a5e9545bcc9c4558fc8a564a8daa2ede997aa6
Lustre-change: http://review.whamcloud.com/9580/

Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I6d7c143713dac809b2f308f7b4f0882345599638
Reviewed-on: http://review.whamcloud.com/9636
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>