LU-544 reduce the journal size for conf-sanity test 56

LU-544 reduce the journal size for conf-sanity test 56

1) reduce the journal size from 16MB to 8MB for reformatting
   the MDT device which has a small size (40MB)
2) speed up filesystem initialization by passing
   "-E lazy_itable_init" to mke2fs

Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I43eeaa1edf43420adeb101ae96b1d9faa36fcbbb
Reviewed-on: http://review.whamcloud.com/1171
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>