[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=1.0.0` allows to run tests in a special manner that is compatibile with 1.0.0 version.

Also moved back randomIndexTemplate to ElasticsearchIntegrationTest (from ImmutableCluster) where all the randomized aspects should be.