LU-8474 tests: stop MGS before setup_noconfig in conf-sanity.sh

LU-8474 tests: stop MGS before setup_noconfig in conf-sanity.sh

In conf-sanity.sh, some sub-tests will leave MGS mounted
under separate MGT and MDT configuration, which will cause
setup_noconfig() fail. This patch fixes the issue by stopping
MGS before running setup_noconfig().

Test-Parameters: trivial combinedmdsmgs=false envdefinitions=ONLY=55 testlist=conf-sanity

Test-Parameters: trivial envdefinitions=ONLY=55 testlist=conf-sanity

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I53f5549c392d300a2c76bbc4ce68e9a8198ba559
Reviewed-on: http://review.whamcloud.com/21653
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Parinay Kondekar <parinay.kondekar@seagate.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>