verify_dynamic_ring: siblings = off

test may fail because of recent default change + systest_read not supporting
siblings. Since I can't recreate it, and it doesn't matter for the test, turn
it off.

Example Failures:
1. http://giddyup.basho.com/#/projects/riak_ee/scorecards/54/54-712-verify_dynamic_ring-centos-6-64-bitcask/23135/artifacts/269380
1. http://giddyup.basho.com/#/projects/riak_ee/scorecards/54/54-612-verify_dynamic_ring-ubuntu-1004-64-bitcask/22859/artifacts/263031
1. http://giddyup.basho.com/#/projects/riak_ee/scorecards/54/54-652-verify_dynamic_ring-ubuntu-1204-64-bitcask/23046

cc/ @jaredmorrow