Add ClamAV

This pull request adds support for ClamAV configuration. ClamAV is already installed and being managed by Puppet (per #8).

scan-backup-data.sh:
- starts clamscan in quiet mode
- scans /srv/backup-data
- logs to /var/log/clamav/clamscan.log
- moves infected file to /srv/infected

The Puppet manifest makes use of fqdn_rand(), with a maximum value of 60, in order to run at a regular, yet random and unpredictable, time.