create a pipeline for building AMIs

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/

Start by reviewing terraform/, terraform/modules/, and terraform/modules/asg/, then compare the linked data.gov infrastructure and immutable-infrastructure article. Define the scope and approach for building AMIs in a pipeline. Done means a working approach is implemented and accepted by the maintainers.

Written by the indexing model from the issue text.