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).
You end up with function calls like this:

```
etcd.Set("/foo", "bar", 0)
etcd.CompareAndSwap("/foo", "bar", "baz", 0)
etcd.CompareAndSwap("/foo", "bar", "", 12)
```

The bare zero and empty strings destroy readability.