LU-11430 tests: get MDC stats by index

LU-11430 tests: get MDC stats by index

Fix test groups 270 and 271 in sanity.sh and
100 in sanityn.sh to get MDC stats using particular
MDC index to prevent multiline output before 'awk'.
In some cases use just 'grep -c' for simpler checks.

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I6bd36192ab800418e3ddb745e128b5ea4d5e20c4
Reviewed-on: https://review.whamcloud.com/33490
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>