Improved AMI search using EC2 image directory.

This improves the `ec2_ami_search` module to search the entire EC2 AMI repository, allowing for things like running Amazon-provided AMIs (NATs, etc.), other third party AMIs (WordPress, etc.) as well as official Ubuntu AMIs as before.

The previous module interface *only* supported Ubuntu images by scraping an Ubuntu-specific datasource.

The new module interface is similar to the earlier one, but not **entirely compatible** (e.g. there is no standard way to search for `distro` or `stream` in the EC2 AMI directory). Examples are given for both Ubuntu and Amazon AMIs.