debian-keyring debian-archive-keyring' for debian-backports takes ages

The provisioning hangs at this point for several minutes:

```
==> gc-gw0: err: /Stage[main]/Ffnord::Resources::Repos/Apt::Source[debian-backports]/Exec[Required packages: 'debian-keyring debian-archive-keyring' for debian-backports]: Failed to call refresh: Command exceeded timeout at /etc/puppet/modules/apt/manifests/source.pp:68
```
