Cache.dpkg_list fails if system locale is not english

For example:

```
# if locale is fr_FR.UTF-8, and git is installed
cache.dpkg_list(['git']) -> {}
```

This is because:

```
# charmhelpers/fetch/ubuntu_apt_pkg.py:158
                pkg.update({k.lower(): v for k, v in zip(headings, data)})
                # pkg = {'nom': 'git', ...
                if 'name' in pkg:
                    pkgs.update({pkg['name']: pkg})
```

Probably need to use a different method to get info from dpkg-query, or force the locale to en_US for consistency.