terraform resources refactorings currently need manual work to prevent deleting existing infras

[gberche-orange]

We would need to automate invocation of "terraform state mv" when the tf configuration changes impact resources that should be preserved on infra.

Background:

• https://ryaneschinger.com/blog/terraform-state-move/
• https://www.terraform.io/docs/commands/state/mv.html

Uses cases:

• moving resources from root module into explicit modules
• renaming modules

Some potential inspirations:

• https://docs.pivotal.io/tiledev/tile-upgrades.html#chaining
• https://flywaydb.org/documentation/migration/
• investigate how the terraform community handles module renames in automation , possibly https://github.com/gruntwork-io/terragrunt/issues/169

[gberche-orange]

To possible way add built-in support could be to manage the refactoring steps as terraform modules that produce bash migration scripts on the local disk using the https://www.terraform.io/docs/providers/local/r/file.html resource, and then invokes them on the terraform state. The tracking of the execution of the refactoring commands might require a distinct tfstate file.

Since our goal is to share this logic with the community as reusable concourse resource, I initiated a related discussion into https://github.com/ljfranklin/terraform-resource/issues/41