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).
why not `return etcd.Set(key, val, ttl)`? This command object pattern seems overkill. You could reduce the amount of LOC I'm sure if you stick to simple functions.