LU-15577 tests: fix interop issue

LU-15577 tests: fix interop issue

Sanity test 831 expects MDS to have osp.*.max_sync_changes
tunable, appeared in 2.14.56.
Adding a check to skip older MDSes.

Fixes: c226e70007 ("LU-15114 osp: changes queuing throttle")
Test-Parameters: trivial serverversion=2.12 serverdistro=el7.9 testlist=sanity env=ONLY=831
Test-Parameters: trivial testlist=sanity env=ONLY=831
Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: I911f7b0d9dd606f08f544fce55bf8bcfe9fb69e3
Reviewed-on: https://review.whamcloud.com/46566
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>