LU-8947 test: fix getting OST name at sanity test_253

LU-8947 test: fix getting OST name at sanity test_253

The test gets OST name the wrong way. And if system
has more than ten OSTs, the test fail. This patch
resolves the issue.

Signed-off-by: Alexander Boyko <alexander.boyko@seagate.com>
Seagate-bug-id: MRP-3330
Test-Parameters: trivial
Change-Id: Ic6080f725c6e693f6cca49d9aebf06c6add4610b
Reviewed-on: https://review.whamcloud.com/24387
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>