Test: Allow global test cluster to have configurable settings source

This allows to reuse the global test cluster with specific configurations,
which is useful in plugins.

Allows to change this in plugin tests like this:

```java
public class FooTest extends ElasticsearchIntegrationTest {

    static {
        InternalTestCluster.CONFIGURABLE_SETTINGS_SOURCE = new SettingsSource() { ... };
    }

    @Test
    public void foo() { ... }
```
not sure about the configurable prefix, maybe `DEFAULT` fits better?