prevent race condition on basho_bench run in yokozuna_essential by incre...

...asing maximum number of rejections

* Test was failing on 2.0.1 GiddyUp

* Fixes race between handoff and cluster metadata (as well as index propagation) as new nodes join the cluster in the `riak_tests/yokozuna_essential` test by raising the [`handoff.max_rejects`](https://github.com/basho/riak_kv/pull/1010/files#diff-6229b4a6ef6df06ebe5d6fb96c06610bR591) configuration for riak_kv.

Fix referenced from changes made on https://github.com/basho/riak_kv/pull/1010 and talking through things w/ @coderoshi, @jrwest, and @seancribbs.