Net: Add `nat.rb`

The `nat.rb` file is a ruleset used to configure NAT rules on the vShield Edge
firewall. This commit adds this file.
Box: Add YAML file for use when launching boxes

The YAML file in this commit is used when launching new GOV.UK off-site backup
machines using `vcloud-launch`.
Box: Add `jenkins.sh` for use when launching boxes

This commit adds a `jenkins.sh` box for use when launching boxes using
`vcloud-launch` and a one-click Jenkinsified installation.