Etcd backend

Added a etcd adapter that watches for key changes under a certain path and adds the backends to the local redis. So etcd is currently not a replacement for redis. It just adds backends to redis automatically.

eg. you can add a backend called 'foo' for the domain giantswarm.io like this.
```
$ etcdctl get /lb/public/giantswarm.io/foo
http://192.168.1.2:49198/
```
ah, ok. then just leave it out.