Update to work with latest riak_ensemble API

[jtuple]

This pull-request updates riak_kv to work with basho/riak_ensemble#10

This pull-request also adds some support API calls to riak_kv_ensembles that are used in some riak_tests as well as can be used for support/debugging.

[andrewjstone]

Seeing the following dialyzer warnings:

riak_kv_vnode.erl:1021: Call to missing or unexported function riak_ensemble_backend:pong/1
riak_kv_ensembles.erl:65: Call to missing or unexported function riak_ensemble_manager:count_quorum/2
riak_kv_ensembles.erl:134: Call to missing or unexported function riak_ensemble_manager:enabled/0
riak_kv_ensembles.erl:153: Call to missing or unexported function riak_ensemble_manager:known_ensembles/0

[andrewjstone]

==> riak_kv (xref)
Query (XC - UC) || (XU - X - B - "(cluster_info|dtrace|yz_kv|yz_stat)" : Mod)
 answer [{{riak_kv_ensembles,bootstrap_preflists,2},
          {riak_ensemble_manager,known_ensembles,0}},
         {{riak_kv_ensembles,count_quorum,0},
          {riak_ensemble_manager,count_quorum,2}},
         {{riak_kv_ensembles,maybe_bootstrap_ensembles,0},
          {riak_ensemble_manager,enabled,0}},
         {{riak_kv_vnode,handle_info,2},{riak_ensemble_backend,pong,1}}]
 did not match []
ERROR: xref failed while processing /Users/ajs/riak_ee/deps/riak_kv: rebar_abort

[andrewjstone]

Code looks good. Once these dialyzer and xref errors are fixed, +1.

[andrewjstone]

When I actually run with the proper riak_ensemble deps I get clean xref and dialyzer.

👍