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

This commit adds the `base::clamav` class to Hiera which is used to autoload the
ClamAV class as we use Hiera as a lightweight ENC in this repository.