Enable timestamps in fleetctl debug logs

The prefix is placed before the header, which results in the first token being the priority:

```
$ ./bin/fleetctl --debug list-units
INFO 2014/09/02 21:59:40 client.go:353: etcd: sending HTTP request GET http://127.0.0.1:4001/v2/keys/_coreos.com/fleet/machines?consistent=true&recursive=true&sorted=true
```

problem?
Why revert the const?