create a pipeline for building AMIs

[afeld]

There is probably one out there already we can steal. Maybe data.gov?

• https://github.com/GSA/datagov-infrastructure/tree/master/terraform
• https://github.com/GSA/datagov-infrastructure/tree/master/terraform/modules
• https://github.com/GSA/datagov-infrastructure/tree/master/terraform/modules/asg
• https://sgotti.me/post/terraform-immutable-infrastructure-stateful-rolling-upgrades/