Test new MR Search Switch and "index" spec

This PR replaces #373.

As of basho/riak_kv#664, Riak attempts to choose the correct system (riak_search or yokozuna) to provide MapReduce inputs when "search" is specified. The `mapred_search_switch` test makes sure that the expected system is chosen, given the cluster's configuration.

As of basho/yokozuna#174, when YZ is chosen, the `Bucket` in the traditional `{search, Bucket, Query}` form of search MR inputs will be translated to its associated index. The three extra commits in this PR that did not appear in #373 test that this is the case, and also that YZ filters results from other buckets stored in the same index.