Support index parameter for Solr Cloud configset name

Currently when JanusGraph creates a Solr collection it either uses the collection name or, if `index.search.solr.configset` is set, a global name for the Solr config set. Goal is to support the config set name as a parameter when creating the mixed index using Solr. Among other things this would allow for restoring support for dropping rather than just clearing Solr Cloud collections (see https://github.com/JanusGraph/janusgraph/pull/563#discussion_r141793273).

Trace how JanusGraph creates mixed indexes and Solr collections, then inspect the Solr Cloud configset handling described in the issue and the linked pull request discussion. Done means the config set name can be supplied as a parameter when creating a mixed index, while preserving the intended collection-dropping support.

Written by the indexing model from the issue text.