CI: validate and check Terraform configuration

[0pdd]

The puzzle 1000-7b4c3a87 from #1000 has to be resolved:

mystamps/infra/terraform/my-stamps.tf

Line 1 in 45b9537

# @todo #1000 CI: validate and check Terraform configuration

The puzzle was created by @php-coder on 11-Jan-20.

[php-coder]

We can run terraform fmt -check -diff and terraform validate.

[php-coder]

See also https://github.com/hashicorp/setup-terraform