Create `firstrun()` function

The firstrun() function sets the user to `ubuntu`, then:
- amends the sudoers file to temporarily allow passwordless sudo
- runs the deployment script

This commit also amends documentation in README.md to reflect the changes needed
to be known when deploying either for the first time or successive times, and
provides a better explanation for running the successive deploy task.
Fix Fabric command
Merge pull request #32 from alphagov/68674340-fabric-fix

Create `firstrun()` function