kubelet: add --runonce flag

### Usage
```
kubelet --runonce --config some/pod-manifest.json
```

### Description
The kubelet runs the pod specified by the manifest and exits when the containers are running.

## Related
#246, #490

### TODO
- [ ] add runonce_test (after I figure out why the kubelet test are broken for me)
did you know you can omit the division by time.Second and go will print with units, e.g. "1s"

http://play.golang.org/p/3JATnggVRk