LU-4433 tests: fix mds-survey.sh to support multiple MDTs

LU-4433 tests: fix mds-survey.sh to support multiple MDTs

This patch fixes mds-survey.sh and mds-survey to support
multiple MDTs.

Test-Parameters: mdtcount=1 testlist=mds-survey
Test-Parameters: envdefinitions=PTLDEBUG=-1,DEBUG_SIZE=150 mdscount=2 mdtcount=4 testlist=mds-survey
Signed-off-by: Fan Yong <fan.yong@intel.com>
Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I9193b7fb65ab8b5dfd0817a8c203dae463deb090
Reviewed-on: http://review.whamcloud.com/19437
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>