dynamic inventory should use Ansible features and not AWS CLI

[thoraxe]

https://github.com/sborenst/ansible_aws_deployer/blob/master/ansible/dynamic_inventory.yml

We should be using Ansible's built-in dynamic inventory for handling inventory instead of running the AWS CLI directly.

Specifically, for EC2:

http://docs.ansible.com/ansible/intro_dynamic_inventory.html#example-aws-ec2-external-inventory-script