New module: ec2_ubuntu_ami

This module will retrieve the AMI of the most recent official Ubuntu
EC2 image for a given Ubuntu release.
Rename to ec2_ami_search
Genericize module to support multiple distros

Make the module implementatino more generic to support distributions
other than Ubuntu in the future.

Adds distro as a new parameter.
Merge pull request #4845 from lorin/ec2_ubuntu_ami

New module: ec2_ami_search