ec2_asg: Implements the proper method for terminating an auto scale group.

The current ec2_asg delete method deletes instances in the group, but then the group attempts to respawn them, resulting in errors such as:

```
  <Error>
    <Type>Sender</Type>
    <Code>ResourceInUse</Code>
    <Message>You cannot delete an AutoScalingGroup while there are instances or pending Spot instance request(s) still in the group.</Message>
  </Error>
  <RequestId>81ac1556-1e76-11e4-bc9b-4deda0593d7f</RequestId>
</ErrorResponse>
```

This pull request [follows Amazon's documentation](http://docs.aws.amazon.com/AutoScaling/latest/APIReference/API_DeleteAutoScalingGroup.html) for deleting AutoScale Groups:

```To remove all instances before calling DeleteAutoScalingGroup, you can call UpdateAutoScalingGroup to set the minimum and maximum size of the AutoScalingGroup to zero.```