[Test] ElasticsearchIntegrationTest.clearDisruptionScheme should test if the current cluster is internal

When running on a non-internal cluster the function is a noop.
[Test] change the default port base for ClusterDiscoveryConfiguration.UnicastZen to 30000

The previous value of 10000 collided with the standard test cluster ports when 6 or more JVMs are used.
fix typo