Add Fabric support

This commit adds details on deployment of the Puppet repo to the readme, along
with an explanation where required.

It also adds a Fabric file from which the Puppet repository can be deployed. The
Fabfile adds a task which bodges the sudoers file in order to allow rsync to run
successfully without a TTY or a requiring an askpass helper to prompt for a
password.

A future commit will have Puppet revert the changes to /etc/sudoers once the
deployment has completed.
Merge branch 'add-fabri' of github.com:alphagov/govuk_offsitebackups-puppet into add-fabri