Box packaging

# metadata.json

```json
{
  "provider": "lxc",
  "version":  "1",

  "template-opts": {
    "--arch":    "amd64",
    "--release": "quantal"
  }
}
```

| KEY                   | REQUIRED? | DESCRIPTION |
| ---                   | ---       | ---         |
| `provider`            | Yes       | [Required](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/box_collection.rb#L31-L33) by Vagrant |
| `version` | Yes       | Manifest version, not really useful at the moment but might be in case we add backward incompatibilities |
| `template-opts`       | No        | Extra options to be passed to the template script provided with the .box package |

# Expected `.box` conteints

| FILE            | DESCRIPTION |
| ---             | ---         |
| `lxc-template`  | Script responsible for creating and setting up the container (used with `lxc-create`) |
| `rootfs.tar.gz` | Compressed container rootfs tarball (need to remeber to pass in `--numeric-owner` when creating it) |