LU-7497 tests: Fix test failure in conf-sanity 32b

LU-7497 tests: Fix test failure in conf-sanity 32b

It turned out that the MGS svc was wrongly turned on in disk image
'disk2_4-zfs.tar.bz2', which caused the OST mount failure with
EALREADY because the MGS service has already been started by the MDS.

This is just a temporary solution and the final fix would be to clean
the disk image in question.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I912d39c08ba1437078fe7126d4498950ebeadaf1
Reviewed-on: https://review.whamcloud.com/25940
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>