Add Fabric support

This pull request adds a Fabric script, with an associated readme explaining its use, and installs the `alphagov/puppet-sudo` module in order to revert changes made by the Fabric script to `/etc/sudoers`. As the sudoers file has a particular security focus, it is important that we are dictating the content of the file.

The `alphagov/puppet-sudo` module we are using does not rely on any further configuration as its default behaviour is to:
- purge any existing sudoers rules
- provide all in the 'sudo' group with wheel privileges

The Fabric script is very similar to that used in [govuk_mirror-puppet](http://github.com/alphagov/govuk_mirror-puppet), except:
- the repository has changed on L13
- the hosts have been changed to point to off-site backup machines in the array on L20
- the IP for the Vagrant VM from the Vagrantfile has been changed on L29
- L39-41 contain a way to temporarily escalate privileges for the ubuntu user when running rsync. The `ubuntu` user is removed by Puppet at first run (see above).