LU-1187 tests: sanity fixes for multiple MDT

LU-1187 tests: sanity fixes for multiple MDT

1. test_27y: deactive/degrade OSCs on all of MDTs, the original
implementation just assume there will be only 1 MDT.
2. test_39l: set atime_diff on MDT0 during the test.
3. test_129: set max_dir_size on all of MDTs.
4. remove test_55bb which is invalid for current DNE.

Signed-off-by: wang di <di.wang@whamcloud.com>
Change-Id: Idc31b078afd23f9834de2218c88e8167785ea854
Reviewed-on: http://review.whamcloud.com/4523
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>