Add Fabric support

This commit:
- adds support for amending the sudoers file automatically on first deploy
- adds support for deploying the application after first deploy
- adds a README to explain usage of the Fabric deployment script
Add alphagov/puppet-sudo module
Remove unused puppet_merge_hieradata function

The `puppet_merge_hieradata` function is used to merge secrets from deployment
repositories at Puppet runtime with Puppet code in public repositories. Since
this repository has no secrets stored in a private repository, then this
function is essentially moot. This commit removes it from the `deploy` task and
removes the function.
Merge pull request #23 from alphagov/add-fabri

Add Fabric support