Declare `cleanup` stage in Terraform ci templates

This change adds the by-default unused cleanup stage to the Terraform CI templates.

The reason for this is that the base templates, actually prepare a .terraform:destroy job which is assigned to this very cleanup stage.

With this change a user can easily enable that destroy job by configuring the pipeline like this:

include:
- template: Terraform.latest.gitlab-ci.yml

destroy:
  extends: .terraform:destroy

Before this change you would get an error that the chosen job stage is not supported.

It's not an issue if the cleanup stage is not used by any jobs.