Feature: Upgrade to Terraform 0.12

[carbonrobot]

There are breaking changes between Terraform 0.11 and Terraform 0.12. I know there is a discussion item for the minimum version to support in #19 but the upgrade may make that issue obsolete.

https://www.terraform.io/upgrade-guides/0-12.html

[kevinmstephens]

The tool provided to check for breaking changes shows nothing (run through the reference app).

STAGE=sandbox aws-vault exec FIRST.LAST --no-session --   yarn tf:service:plan
yarn run v1.21.1
$ yarn run tf:terraform 0.12checklist $(yarn -s tf:service:_vars)
$ eval $(yarn -s env) && cd terraform && terraform 0.12checklist -var region=us-east-1 -var service_name=simple-reference -var stage=sandbox
Looks good! We did not detect any problems that ought to be
addressed before upgrading to Terraform v0.12.

This tool is not perfect though, so please check the v0.12 upgrade
guide for additional guidance, and for next steps:
    https://www.terraform.io/upgrade-guides/0-12.html