LU-3802 tests: only start mdt0 for conf-sanity 68

LU-3802 tests: only start mdt0 for conf-sanity 68

Only start MDT0 for conf-sanity 68, otherwise other
MDT might reserve sequence as well, which will confuse
test result.

Test-Parameters: mdtcount=2 mdscount=2
Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I27b583480ce608cad7743877d14f3f372d7ed645
Reviewed-on: http://review.whamcloud.com/7407
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>