LU-3369 scrub: skip test_1a for old 2.x (x <= 3) release

LU-3369 scrub: skip test_1a for old 2.x (x <= 3) release

Lustre-2.3 or older does not support initial OI scrub,
so skip related sanity-scrub test_1a.

Test-Parameters: testlist=sanity-scrub
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I17752be675bdda76588c55c38c20c37a009eff00
Reviewed-on: http://review.whamcloud.com/6417
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>