Test: Allow global test cluster to have configurable settings source

[spinscale]

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:

public class FooTest extends ElasticsearchIntegrationTest {

    static {
        InternalTestCluster.DEFAULT_SETTINGS_SOURCE = new SettingsSource() { ... };
    }

    @Test
    public void foo() { ... }

[s1monw]

awesome maybe we can drop that extra ctor in InternalTestCluster too then?

[javanna]

maybe package private instead of public?

[spinscale]

@s1monw I removed the simple unused constructor and left the other two in place. The TribeTests are using it as well and doesnt make too much sense for them to set it explicety in the tests.. this way everything can stay package private

[javanna]

Left a minor comment, other that that looks great to me!

[nik9000]

Sweet! I'm looking forward to this!

[javanna]

Hey @nik9000 right this would help #7040 too wouldn't it?

[nik9000]

@javanna yeah! It pretty much closes that issue.

[s1monw]

LGTM