How to run database migrations in an auto-scaled group
How are you doing your deploys?  Nuke + rebuild the ASG (whether one node at a time, or by replacing the whole group at once), or do you have a script that redeploys all of your active nodes?

And also, how are you triggering your deploys?

Ideally you want to be running/triggering deploys from some form of CI server like Bamboo or Jenkins.  If you're doing this, you can run your DB migrations from the build server (or its component build agent instances).

This would allow you greater control over your environment, as you could, for example, restore DB from snapshot if the deployment fails.