LU-8701 tests: scrub-performance with separated MGS and MDS

LU-8701 tests: scrub-performance with separated MGS and MDS

Support to run scrub-performance under the environment of MGS and
MDS on separated devices.

Test-Parameters: trivial envdefinitions=SLOW=yes combinedmdsmgs=false testlist=scrub-performance
Test-Parameters: trivial envdefinitions=SLOW=yes testlist=scrub-performance
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I5b01097f3e73c2e0eca6071ecb597ac84893c77d
Reviewed-on: https://review.whamcloud.com/23124
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>