Create a public (shared) Hadoop EC2 AMI

HADOOP-884 makes it easy to run Hadoop on an EC2 cluster, but building an AMI (Abstract Machine Image) can take a little while. Amazon EC2 supports shared AMIs (http://developer.amazonwebservices.com/connect/entry.jspa?entryID=530&ref=featured), so we could provide publically available AMIs for each Hadoop release.