For the benefit of others that might come across this question searching for why their ECS task/service might be stuck on PROVISIONING with CloudFormation on CREATE_IN_PROGRESS:

Make sure you haven't re-used a launch template from a different ECS cluster. As the other answers point out: there is a bash script in the Advanced section at the bottom of the template creation page that contains information specific to one cluster. You can re-use it, but you must change the cluster name. You will know if you have it set up correctly if the instance appears in the Infrastructure tab in the cluster on ECS.