CI: validate and check Terraform configuration

[0pdd]

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

https://github.com/php-coder/mystamps/blob/45b953747ede2a2899ef5825ab89dd516d59f137/infra/terraform/my-stamps.tf#L1-L1

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