Adding DKIM record for cucumber's public key

The key is located in /etc/exim4/dkim-cucumber.key and .pub and
they are not a part of the puppet definition.
taken as-is from the current /etc/exim4/update-exim4.conf.conf
On 2nd thought, there's nothing cucumber specific about this.

This configuration would be appliable to any hosts that route e-mails by themselves.
split exim4 config into DKIM portion and self-routing portion
need to load local modules as well.

Does this mean no one has been testing stuff with Vagrant!?
fixing up loose ends with exim4 configuration
let's also generate key in puppet as well

... since we are doing per-host key
applying DKIM setting on cucumber