Allow overriding the default Elasticsearch index when using the external connector

[mattflax]

Currently the external Elasticsearch connector only allows connecting to the index specified in the ratings file. If we have changed our mappings/analysis and written to a new index, it would be useful to test that index alongside the previous version.

The external ES archetype currently has an index entry in the index-settings.json file which is ignored. This feature request should allow that value to be different from the value in the ratings file, and for that index to be used.