New cluster/handoff CLI coverage

Add CLI test coverage for the new handoff and cluster commands added in riak_core from basho/riak_core#659 basho/riak_core#660 and basho/riak_core#661.  These commands include

* handoff enable|disable
* handoff summary
* handoff details
* cluster status
* cluster partitions
* cluster partition-count
* cluster partition id|index
* cluster members

There is also coverage for the "set / show / describe" cuttlefish configuration values in riak_cli.