New module: ec2_ubuntu_ami

This module will retrieve the AMI of the most recent official Ubuntu EC2 image for a given Ubuntu release.

Example usage:
```
- name: Lauch an Ubuntu 12.04 (Precise Pangolin) EC2 instance
  hosts: 127.0.0.1
  connection: local
  tasks:
  - name: Get the Ubuntu precise AMI
    ec2_ubuntu_ami: release=precise region=us-west-1 store=instance-store
    register: ubuntu_image
  - name: Start the EC2 instance
    ec2: image={{ ubuntu_image.ami }} instance_type=m1.small key_name=mykey
```