Replace go-etcd

This replaces the dependencies on github.com/coreos/go-etcd and github.com/coroes/etcd with a new local package surprisingly called "etcd". It is only intended to implement the functionality that fleet needs (for now).
I would suggest also testing for keys that contain URL-escapable symbols, like %.  I wrote https://github.com/coreos/go-etcd/pull/137 to cover this scenario for go-etcd.