Add ClamAV manifest; add class to Hiera

This commit adds a manifest (clamav.pp) to Puppet in order to control ClamAV
(http://www.clamav.net). The manifest itself does not directly run ClamAV.
Rather, it contains a cron type to run a shell script at a random number of
minutes (between 0 and 60) past every hour. The shell script's functionality is
covered in da09c66.

This commit also adds the appropriate class instantiated within the Puppet
manifest to Hiera such that this class can be autoloaded using Hiera as a
lightweight ENC.
Adds script to scan /srv/backup-data

This is a script to scan /srv/backup-data, where backed-up data lives. It works
in conjunction with ab42f22 to scan data backed up from source GOV.UK nodes.

Firstly, it echoes the fact that it's going to start running to clamdscan.log,
and then runs clamscan in quiet mode, logging to clamdscan.log, on
/srv/backup-data. Any infected files are moved to /srv/infected.
Adds alphagov/puppet-clamav module to Puppetfile

In order for either ab42f22 or da09c66 to work, this commit adds the
'puppet-clamav' module from the alphagov Git organisation
(http://github.com/alphagov/puppet-clamav.git) to the Puppetfile.