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.
Add a README for configuring networking

A README for configuring networking. Very useful if you're new to the repository
and want to know how things work.
Amend BACKUP_SERVER_IPS values

There will only, currently, be one off-site backup server on Carrenza. Thus, the
IPs have been changed and the variable name has been changed from `...IPS` to
`...IP`.

It has been left variabalised in order to allow for additional IPs to be added
with limited overhead in future.
Set Gem install path

By setting the Gem install path, we can ensure that Bundler installs Gems
outside of the Jenkins workspace. Thus, when builds are running, they can pull
in Gems saved outside of the workspace (which is trashed after every job). This
improves how quickly jobs run at buildtime.
Amend firewall rules

This commit:

- fixes the GOVUK_PROD_IP variable:
-- adds a trailing closing bracket
-- adds the P1 production SNAT IP

- removes excess or unwanted firewall rules
- changes comments and variables to reference `backup` rather than `mirror`
Add load-balancing support

Whilst we don't currently use a load-balancer to balance requests to the
off-site backup machine, I am adding support for load-balancing for two reasons:

1) We may need to load-balance in future, and adding support at this stages
   reduces overheads in future when we need to start balancing requests.

2) If load balancing is configured manually on the vSE (which it shouldn't be,
   but it may well happen), we will not have a version-controlled configuration.
   By providing a configuration, we can at least ensure that Jenkins is applying
   the version-controlled configuration at all times.

This commit, therefore, adds:

- the `lb` component to the `jenkins.sh` file in order to configure
  load-balancing from `vcloud-net-launch`

- a ruleset to apply using `vcloud-net-launch`