LU-4058 tests: skip sanity-scrub test_15 for 2.4.1 or older

LU-4058 tests: skip sanity-scrub test_15 for 2.4.1 or older

Lustre-2.4.1 or older does not support dry-run mode OI scrub yet.

Test-Parameters: mdsjob=lustre-b2_4 ossjob=lustre-b2_4 \
mdsbuildno=54 ossbuildno=54 testlist=sanity-scrub
Signed-off-by: Fan Yong <fan.yong@intel.com>
Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I1a97f850ab39d7d42ef7ed6e191ce66b3b093415
Reviewed-on: http://review.whamcloud.com/7868
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>