Depricating terraform_modules option since no longer used

[costrouc]

Depricating terraform_modules option since no longer used. This was part of the original PR in #957 .

[danlester]

Also need to add this to 0.4.0-specific upgrade step in qhub/upgrade.py:

        if "terraform_modules" in config:
            del config["terraform_modules"]
            print(
                "Removing terraform_modules field from config as it is no longer used.\n"
            )

[danlester]

I've added this now.