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).
Empty strings **do not** destroy readability. They increase it in this case. They show intent. You could add a few sugar methods to wrap things like clobbering a key. Call it, say `Clobber`. That tells me, the future reader, even more.