Arch image cloud-init generate a wrong locale.gen file

cloud-init generate not correctly /etc/locale.gen, like this : 

```
# Created by cloud-init v. 0.7.6 on Thu, 03 Mar 2016 06:25:03 +0000
LANG=en_US.UTF-8
```

This should be

```
en_US.UTF-8 UTF-8
```

LANG=en_US.UTF-8 is valid only for **/etc/locale.conf** I think the confusion comes from here.
