Elasticsearch2 replace Kopf with Cerebro

[rene-demonsters]

As mentioned on the repository of Kopf, Kopfs is no longer being maintained: https://github.com/lmenezes/elasticsearch-kopf.

The developer has been working on a replacement in the form of Cerebro: https://github.com/lmenezes/cerebro.

The Elasticsearch2 catalog image should probably be updated to keep up with this development as it still comes with Kopf right now.