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