Add dns name support to ec2

Hello!

I would like to add dns_name feature to ec2 module.

For creation instances, dns_name parameter will work like id parameter, but in more convenient way. If instance with target dns_name exists, then the new instance with the same dns_name will not be created.

For termination instances, dns_name parameter will work like instance_ids parameter.

create:
    - name: Launch instance
      local_action: ec2
        dns_name={{ item.dns_name }}
        group={{ item.group }}
        ...
        aws_secret_key={{ aws_secret_key }}
        wait=true
      register: ec2

terminate:
tasks:
    - name: Terminate instance
      local_action: ec2
        dns_name={{ item.dns_name }}
        ...
        aws_secret_key={{ aws_secret_key }}
        state='absent'
      with_items:
          - dns_name: ec2-184-169-125-4.us-west-1.compute.amazonaws.com
          - dns_name: ec2-158-120-200-124.us-west-1.compute.amazonaws.com