LU-7222 tests: add Mulitple MDTs to test_84

LU-7222 tests: add Mulitple MDTs to test_84

Add multiple MDTs to conf_sanity.sh test_84(), and
add more information into the error message when
config log corrupted.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I45160d053f8dd52ca3230888e720fc04102d50ab
Reviewed-on: http://review.whamcloud.com/16662
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>