Add alphagov/clamav module to Puppetfile

Adds the alphagov/clamav module to Puppetfile. We're eating our own dog food here.
Add `base::clamav` class to Hiera

To use the alphagov/clamav package introduced in 5e9cfe3, we need to add it to Hiera as we're using Hiera as a
lightweight ENC in this instance.
Add /srv/infected directory

The /srv/infected directory is used by ClamAV to move infected files to in
7660f5d. This change ensures that the directory exists.
Add shell script to scan /srv/backup-data

This commit adds a shell script to scan the contents of /srv/backup-data, log
output to a specified .log file, and move infected data to /srv/infected
Add clamav.pp manifest to control ClamAV scanning

This commit adds a manifest (clamav.pp) to Puppet in order to control ClamAV.
The manifest itself does not directly run ClamAV, but rather it contains a cron
type to run a shell script at a random number of minutes (between 0 and 60) past
every hour.