Refactor Discovery Backend Registration

[justenwalker]

As we are going to support multiple backends, it would be good if we didn't have to alter global config parser so much.

I propose that we make the config more free-form (map[string]interface{}) and support hooking into keys using a Registration function. Discovery backends could call this registration in their func init() { ... } to register a handler for their key ("consul", "etcd", "zookeeper", ...)

I'll try an initial PR to get the idea out in a more concrete form. I think this will significantly improve how we parse the configuration file.

As a part of this enhancement, it would be good to push the discovery backends into their own package.

[tgross]

Merged in #144. I'm building a release for this now.

[tgross]

Released in https://github.com/joyent/containerpilot/releases/tag/2.1.1