LU-7529 test: fix tiny problems of tests in conf-sanity.sh

LU-7529 test: fix tiny problems of tests in conf-sanity.sh

1)test_68:
Mount MGS in the end of test if MGS and MDS are seperated.
2)test_75:
Reformat all devices otherwise 76a might fail.

Change-Id: Iee8a5fa1c6acccbae86cdcbe14c8715a71b0ae2b
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: teddy <jjkky@yahoo.com>
Reviewed-on: https://review.whamcloud.com/17507
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>