Add ping_quorum and get_peer_info support calls

`riak_ensemble_peer:ping_quorum/1` checks an ensemble for an active
quorum, returning the current ensemble leader as well as the list of
peers supporting the current quorum.

`riak_ensemble_manager:get_peer_info/2` returns information about a
given ensemble peer: the peer's current FSM state, if it is or is not
trusted, and it's current epoch.

Sibling pull-request: basho/riak_kv#979

It didn't really sneak in. That line was in the original code as shown in the diff. Didn't want to clutter PR with changes unrelated to the actual change made.