Allow overriding the default Solr collection when using the external connector

[mattflax]

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

The external Solr archetype currently has a collectionName entry in the solr-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 collection to be used.