LU-3332 tests: Add DNE support to sanity-scrub

LU-3332 tests: Add DNE support to sanity-scrub

sanity-scrub only exercises MDT 0.  This patch enhances it to also
test OI scrub on the other MDTs, if available.

Test-Parameters: mdtcount=2 testlist=sanity-scrub
Change-Id: Ic11abf964a85892f3c772cc1a12f3e94c8f2a256
Signed-off-by: wang di <di.wang@intel.com>
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/6333
Tested-by: Hudson
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>