feature(network-online.target): ensure IP routable before enabling

This implements `network-online.target` by waiting until there is a route to `8.8.8.8` (arbitrarily-chosen public IP). It changes `coreos-setup-environment.service` to run after `network-online.target`.

Where previously my environment's public IP would be `0.0.0.0`:

```
core@localhost ~ $ cat /etc/environment
COREOS_PUBLIC_IPV4=10.0.2.15
COREOS_PRIVATE_IPV4=127.0.0.1
```

![img](https://d2oawfjgoy88bd.cloudfront.net/533da756c38aa51217f640dd/533da777c38aa51219a9875c/537e937cc38aa547518df544.png?Expires=1400890623&Signature=RF1KeVFH0oO5l6nq28OG1z5MjsZioZY1mz6QhLlrRAngfnsqZsoc~KekkgPWgi-JW71oF~E~p89NSaLHSJ3aGrNwVqTh1qa8faav3bHSwubT6wHJFncsIh~2p72~7i5agnxlvBA40AA3Q59bkzj~QqMJz50RpMMznHeABhEVvRwpRQHiDdXxCCDHm7R2w18ywbGcWc1B4nAHSEO5VzFcMS79CoYTrI6V9uCmpSEEC5~UVj3wWaMWMWGXplXYI1WR59NxvS7yOADduX9VUtpY9qfJOZT7EvbU3Vi8K5Vi8JWWAmTTOuDLxtNxzBsPSxWS9Y7UstVYQQgtlmrTiO3Tyw__&Key-Pair-Id=APKAJHEJJBIZWFB73RSA)

```
 -- Logs begin at Fri 2014-05-23 00:11:18 UTC, end at Fri 2014-05-23 00:14:08 UTC . --
May 23 00:11:33 localhost systemd[1]: Starting Wait for Network...
May 23 00:11:33 localhost wait_for_network[2998]: interface not up after 0 ms...
May 23 00:11:33 localhost wait_for_network[2998]: interface not up after 100 ms...
May 23 00:11:33 localhost wait_for_network[2998]: interface not up after 200 ms...
May 23 00:11:33 localhost wait_for_network[2998]: Got routable IP 10.0.2.15 after 300 ms.
May 23 00:11:33 localhost systemd[1]: Started Wait for Network.
```


Other services that should also be waiting on this that spring to mind:

* Whatever fetches remote cloudinit files.