HalfAtATime deployment with an ELB/ASG

We currently use these scripts during deployment to deregister our instances from an ASG during deployment. However, we've discovered what appears to be a race condition when using HalfAtATime deployment in CodeDeploy and using more than 2 EC2 instances in the ASG.

For example, if you have 4 EC2 instances behind an ASG and run a HalfAtATime deployment on them from CodeDeploy, it will run the deployment scripts against 2 of the 4 at the same time. From there, one of them will instantly fail citing:

```
[stderr]An error occurred (ValidationError) when calling the EnterStandby operation: AutoScalingGroup [DeploymentGroup] has min-size=3, max-size=70, and desired-size=3. To place into standby 1 instances, please update the AutoScalingGroup sizes appropriately.
```

I believe this is because both EC2 instances are running `autoscaling_enter_standby()` at the same time. So they both scale the ASG min and desired down to 3 at the same time and then attempt to move into standby, however only one gets in. When the other makes the attempt, it fails because by that time there are 3 in the ASG and you can't take another one away.

Is there any way to adjust the script to account for this issue? Without that, HalfAtATime deployment is pretty useless given that you're limited to just 2 instances in the ASG at a time.