Add `puppetlabs/apt` module to Puppetfile

Add `puppetlabs/apt` module, which allows us to use unattended-upgrades on this
box.
Add `mail_to` key to Hiera

This commit adds a `mail_to` key to Hiera in order to direct exception mail from
apt to the machine.email list.
Merge pull request #18 from alphagov/add-apt

Add `apt`, configure `unattended-upgrades`