ci: Add a ci stage for `terraform validate`

By adding a test-validate job in the ci project we could have anticipated the bug with v0.4.0 and terraform 0.14.2 in #18 (closed).

The failed job in the pipeline is fix if we merge !43 (merged) too.

I think both MR are importants. In both I added the new test-validate job to check the implementation.

test-validate:
  extends:
    - .test
    - .versions
  stage: test-validate
  script:
    - gitlab-terraform validate