Add ensemble_ring_changes

Ensemble_ring_changes tests writing a value, expanding the cluster, then
updating and reading that value after ring expansion has completed. It
also creates a bucket using a bucket type with a different n_val from
the default bucket type.  The latter tests basho/riak_kv#1008 and it's
corresponding riak_core PR.

Additionally it tests replacing and force replacing a single node at a time to ensure reads and writes still work.