Rename public facing Yokozuna to Riak Search 2.0

We need to rename all of the cases where the term yokozuna is public facing, with some variety of `search`, or `rs`, so as to clarify that this is a Riak Search 2.0 project.

- [x] Update HTTP admin resource names from `yz` prefix to `rs`
- [x] Update all yokozuna.schema cuttlefish mappings from `yokozuna` prefix to `search`
- [x] Rename `yz_index` bucket label to `search_index`
- [x] Update docs with use new resource paths and settings
- [x] Rename `yokozuna.*` security to `search.*` (also change `yokozuna.search` to `search.query`)

Testing this relies on basho/riak-erlang-client#134.

After this is merged:
* the mapred test needs updated basho/riak_test#454
* the remaining drivers need the renamed bucket prop `search_index`