Cross cluster aggregations don't include cluster alias in _index

[spalger]

Elasticsearch version: master
Plugins installed: none
JVM version: 1.8.0_121
OS version: macOS

Description of the problem including expected versus actual behavior:
When running a search on a remote cluster, remote documents include the cluster alias in the _index field so that the document can be properly addressed. This behavior does not apply to aggregations. When doing a terms agg on _index for instance, the terms do not include the cluster alias. The top_hits agg also doesn't enhance _index with the cluster alias.

Steps to reproduce:

1. Start a node that remotes to itself

   ./bin/elasticsearch -E search.remote.local.seeds=localhost:9300

2. Index a document

   curl -XPOST "http://localhost:9200/index/doc/id" -H 'Content-Type: application/json' -d'
   {
     "foo": "bar"
   }'

3. Execute a search for the document via local and remote index names, which results in two unique documents (correct behavior) but one index term in the aggs and what appears to be two identical top_hits

   curl -XPOST "http://localhost:9200/index,local:index/_search" -H 'Content-Type: application/json' -d'
   {
     "aggs": {
       "indices": {
         "terms": {
           "field": "_index"
         },
         "aggs": {
           "hits": {
             "top_hits": {
               "size": 10
             }
           }
         }
       }
     }
   }'

   results:

   {
     "took": 7,
     "timed_out": false,
     "_shards": {
       "total": 10,
       "successful": 10,
       "failed": 0
     },
     "hits": {
       "total": 2,
       "max_score": 1,
       "hits": [
         {
           "_index": "index",
           "_type": "doc",
           "_id": "id",
           "_score": 1,
           "_source": {
             "foo": "bar"
           }
         },
         {
           "_index": "local:index",
           "_type": "doc",
           "_id": "id",
           "_score": 1,
           "_source": {
             "foo": "bar"
           }
         }
       ]
     },
     "aggregations": {
       "indices": {
         "doc_count_error_upper_bound": 0,
         "sum_other_doc_count": 0,
         "buckets": [
           {
             "key": "index",
             "doc_count": 2,
             "hits": {
               "hits": {
                 "total": 2,
                 "max_score": 1,
                 "hits": [
                   {
                     "_index": "index",
                     "_type": "doc",
                     "_id": "id",
                     "_score": 1,
                     "_source": {
                       "foo": "bar"
                     }
                   },
                   {
                     "_index": "index",
                     "_type": "doc",
                     "_id": "id",
                     "_score": 1,
                     "_source": {
                       "foo": "bar"
                     }
                   }
                 ]
               }
             }
           }
         ]
       }
     }
   }