Add puppet modules

This pull request adds:
- `puppetlabs/apt` to allow Puppet (and, soon, unattended upgrades) to control package updates
- `attachmentgenie/ufw` to allow for management of ufw firewall rules using Puppet
- `attachmentgenie/ssh` to allow for management of SSH using Puppet
- `blom/rssh` to lock `govuk-backup` user to a shell which only allows rsync
- `alphagov/lvm` to allow configuration of LVM drives via Puppet
- `alphagov/puppet-ext4mount` to allow mounting of ext4 drives via Puppet
- `alphagov/puppet-harden` to harden Ubuntu installation
- `alphagov/puppet-clamav` to install and manage ClamAV via Puppet