gitlab-ci: Add --diff to terraform fmt command

Added --diff to the terraform fmt --check command, because it then will show the
offending lines on the pipeline output.

.gitlab-ci.yml

@@ -19,4 +19,4 @@ terraform-validate:
     - cd ../tf-stage2
     - terraform init -backend=false
     - terraform validate
-    - terraform fmt --check
+    - terraform fmt --check --diff