Add support for AWS Auto Scaling

[bploetz]

AWS Clouds should support Auto Scaling for Ec2 Nodes. This implies:

1. Adding a workflow step to create an AMI + LaunchConfiguration from an already configured Ec2 Node.
2. Updating the ec2_node DSL to decorate an Ec2 Node as being in an AutoScalingGroup.
3. Integration with Elb Nodes when present.