Avoid log flooding with messages about missing missing registry/services

Prevents this to appear in log every 2s:

```
26.314664 13972 etcd.go:118] Failed to get the key registry/services: 100: Key not found (/registry/services) [23]
26.314689 13972 etcd.go:78] Failed to get any services: 100: Key not found (/registry/services) [23]
```
Check for tools.IsEtcdNotFound(err) and return empty arrays (which should be []api.Service{}, not make(...)) without an error.