Associate search index with one bucket

A question came up on the mailing list about associating a Solr index
with one bucket. This can be done by setting `search_index` in the
bucket's properties. When more than one bucket share the same index, a
bucket type should be used.
Merge pull request #952 from basho/search-index-associate-one-bucket

Associate search index with one bucket

Thanks @lucperkins