(RE-91) cowsay in deb enterprise-dist packaging

This commit modified the puppetlabs-debbuilder so that it includes
cowsay if we're provisioning a deb platform to build a pe package.

I'm not sure if adding an entirely new file is too much change. Alternatively, I could pass the $pe variable to $debbuilder::packages::essential, because that will be called if we're using cows or not. However, that again introduces a significant amount of change.