ZookeeperInstanceDiscovery

Related to but not requiring https://github.com/Netflix/Turbine/pull/19 (sets a server-port instance attribute useful in instanceUrlSuffix)

Uses curator-x-discovery to find instances. Under a specific ZK directory (`turbine.ZookeeperInstanceDiscovery.zookeeper.serviceDiscoveryPath`) it will look for every cluster in `turbine.aggregator.clusterConfig`. Uses the ServiceCache around each ServiceDiscovery instance.

Feedback would be appreciated.