Make EC2 image independent of Hadoop version

Instead of building a new image for each released version of Hadoop, install Hadoop on instance start up. Since it is a small download this would not add significantly to startup time. Hadoop releases would be mirrored on S3 for scalability (and to avoid bandwidth costs). The version to install would be found from the instance metadata - this would be a download URL.

More generally, the instance could retrieve a script to run on start up from a URL specified in the metadata. The script would install and configure Hadoop, but it could be extended to do cluster-specific set up.