LU-8333 test: make sure COS is cleared

LU-8333 test: make sure COS is cleared

In subtest 21b of replay-dual, the COS could be set after the MDT
is failed over, and the test will fail in this case

Change-Id: I9401b905593c76f8fddfab19ab9eb6c0fe886e41
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/21924
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>