Use the new etcd client

[vcastellm]

The current etcd client is being deprecated, the new client https://godoc.org/github.com/coreos/etcd/client will have all the focus in the future.

[samgaw]

Would you consider Docker's libkv instead of the etcd client?

github.com/docker/libkv

[vcastellm]

I thought this should be abstracted to allow several storage backends! thanks @samgaw I wanted this to exists.

Definitely worth considering.

From what I see in the docs the transition should be straightforward.

[samgaw]

We moved to Consul so the etcd code was the first thing I skimmed through to see how baked in it was but it's pretty minimal.

If I get a chance over the next couple of days I'll RTFM libkv but I wanted to check first that there wasn't a specific reason for updating the etcd client.

[vcastellm]

@samgaw Look at #21