Expose _cluster on search response

[Mpdreamz]

By default cross cluster searches will fail the whole search if a remote is not available:

https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-cross-cluster-search.html

This is now configurable through

PUT _cluster/settings
{
  "persistent": {
    "cluster.remote.cluster_two.skip_unavailable": true 
  }
}

And will start to return the following block on the search response.

"_clusters": { 
    "total": 3,
    "successful": 2,
    "skipped": 1
  },