[TEST] Minor  REST tests infra cleanup

Make the http addresses within the REST client final. It makes no sense to update them before each test if we don't check the version of the nodes again, which would mean adding too much overhead (an additional http call before each test) for no reason. We just reuse the same nodes for the whole suite and check the version once while initializing the client. Would be nice to make the REST client within the execution context final but its initialization still needs to happen after the `ElasticsearchIntegrationTest#beforeInternal` that assigns `GLOBAL_CLUSTER` to `currentCluster`.

Closes #7723