LU-2628 tests: disable test_40 of replay-single

This test case assumes that IO to OSTs could go on even when the
connect to MDT is lost, this is not true any more because clients
have to verify the layout is correct before operating OST objects.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I08eaad7b97da7ee152c066426f24bc1d15db5738
Reviewed-on: http://review.whamcloud.com/5056
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>