only issue client ops if ensembles enabled

Ready for review
I actually had it that way prior to this and rebased to do the rpc instead. The reason is that theoretically, the client could be used from nodes that don't run riak_ensemble code. I don't think that's an issue with us/riak but it could be for others. In that case, maybe @engelsanchez  is right that this belongs in riak_client instead.