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).
It feels sufficiently intuitive to me for 

```
Delete{
  Key: "/foo"
}
``` 
to perform a Delete, and
```
Delete{
  Key: "/foo"
  PreviousValue: "bar"
}
```
to be a CompareAndDelete