Resolve "Create an integration test"

This does not tests the deploy and destroy jobs, since they're manual. But I believe it's probably good enough, since it already tests terraform plan. I'd push this this forward as a first iteration.

It also doesn't test all the terraform versions in the parallel matrix, as it would require a dynamic child pipeline which is a bit more convoluted. In the iteration spirit, I think it's probably enough to also start simpler doing the integration testing based on the STABLE_VERSION.

Closes #70 (closed)