This repo contains a cloud-init script for easy reactor installation.
Currently this script supports automated reactor installation on Ubuntu 12.04 (LTS).
To use it, simply edit the cloud-init file and change ${KEY} to your repository key.
(You could also use sed or other such trickery to make the change.)
Then, pass the cloud-init file as user data when you boot your instance, for example:
nova boot --user-data=cloud-init --flavor=1 reactor