Introduces encrypted, incremental, off-site Amazon s3 backups using duplicity

When set up, you'll need to set up 3 parameters in common_hiera_secure.json:

```json
{
    "duplicity::params::dest_id": "<Amazon Client ID>",
    "duplicity::params::dest_key": "<Amazon Secure Key>",
    "duplicity::params::pubkey_id": "<GPG public encryption key>"
}
```