setup-environment needs to run after network.target

I get this with the current alpha, installed to disk on bare metal:

```
core@core0001 ~ $ cat /etc/environment
COREOS_PUBLIC_IPV4=0.0.0.0
COREOS_PRIVATE_IPV4=127.0.0.1
```

```
-- Reboot --
May 20 20:32:31 core0001 systemd[1]: Starting Modifies /etc/environment for CoreOS...
May 20 20:32:31 core0001 coreos-setup-environment[3290]: RTNETLINK answers: Network is unreachable
May 20 20:32:31 core0001 systemd[1]: Started Modifies /etc/environment for CoreOS.
```

I believe adding network.target as a prereq here will cause it to stop failing.