Adding possibility to add admin_port value, and also report it to ZK.

We are evaluating SmartStack (Nerve/Synapse/HAProxy) in our products, and since we are using Dropwizard/Metrics extensively we need to be able to do the checkon  another port. 

Now this is already possible by override the port in the checks, but I also need to make the admin port available in Zookeeper as we are building a service to keep track of all running services and their state. To do that we need to get hold of the admin port in addition to the regular traffic port.

Not a daily rubyist, so hopefully my coding is up to your standards. If it is not, I will not be offended :-)

Hopefully this scratches someone elses back as well.