[TEST] Allow to disable randomization of shards and replicas via system property

Needed for REST backwards compatibility tests, since we need to run older tests with the latest runner, which randomizes shards and replicas, but the tests rely on defaults (5,1).

Done in a generic way based on compatibility versions e.g. `-Dtests.compatibility=default_num_shards_replicas` allows to disable the randomization of number of shards and replicas in the tests runner.