Add an alternate service discovery backend

[tgross]

The PR #4 provided by @bbox-kula made the changes we needed to have a generic interface for service discovery so we can have pluggable backends as intended. Including a second implementation of this backend as an example would make sure this works out in practice.

TODO on this issue: pick a second backend

[stonevil]

+1

I'm interesting to add etcd backend support.

[tenfourty]

+1 on etcd as the next backend to be supported.

[tgross]

@stonevil did you want to take that on?

[bbox-kula]

@stonevil I've got some ideas, if you wanted to bounce ideas off of someone. Otherwise, after things settle down here I'd take a crack at adding it.

[tgross]

Released in https://github.com/joyent/containerbuddy/releases/tag/0.1.0