1:M index:bucket mapping

Allow a one-to-many index-to-bucket mapping.  This allows splitting data into logical buckets without requiring the use of multiple Solr Cores.

* Change the index content flag to instead be the name of the index for which the bucket indexes under.  I.e. change the bucket property name from `yz_index_content` to `yz_index`.

* The bucket property is already being pulled during index time so just change the index hook to use the value as the index to write to.