cannot drop janusgraph_vertices index when using external mappings.

I'm using cassandra 3.11 and ES 5.x with janusgraph 0.2.1.

I open graph with the following two parameters configured and then call `JanusGraphFactory.drop`.

```
baseConfiguration.setProperty("index.search.elasticsearch.use-all-field", false);
baseConfiguration.setProperty("index.search.elasticsearch.create.use-external-mappings", true);
graph = JanusGraphFactory.open(baseConfiguration);
...
JanusGraphFactory.drop(graph);

```
Only keyspace janusgraph being removed.
Is it correct to remove janusgraph keyspace and janusgraph_vertices index manually?

Reproduce the behavior with Cassandra 3.11, Elasticsearch 5.x, JanusGraph 0.2.1, and the two external-mapping properties shown in the issue. Start at JanusGraphFactory.drop and determine whether removing the janusgraph keyspace should also remove the janusgraph_vertices index; done means the expected cleanup behavior is established and covered by the relevant test or documentation.

Written by the indexing model from the issue text.