Allow management of the Zookeeper service.

As a first pass towards #86, this PR adds the ability to examine a new `node[:zookeeper][:service_style]` attribute and either create a Runit service, or allow Exhibitor to manage Zookeeper. It doesn't do anything special for Exhibitor; that is a decision to revisit later (perhaps a suggests in metadata for Exhibitor). For now, it'll log a different message, just to indicate this is a "valid" value.