LU-3852 tests: disable test_251 from inside sanity-hsm

LU-3852 tests: disable test_251 from inside sanity-hsm

Due to test_251 frequent failures, subtest has been disabled at
the autotests config level. To be able to work on a patch to fix
the root cause (likely to be a pure consequence when running on
a too small-sized Lustre filesystem), subtest will be disabled
from inside sanity-hsm to allow autotests config changes to be
reverted and to re-enable it on a per-patch basis.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I7dfae4d278cba049062480c276f9cad10d22e0e9
Reviewed-on: http://review.whamcloud.com/9014
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Andreas Dilger <andreas.dilger@intel.com>