Consider updating Terraform util script so user must specify JSON file extension

[ben-moss]

In the util script changes added in #298 the user now has the ability to specify which template they want to apply/destroy. In the current version, the user just specifies the "name" of the template (e.g. single_node_hana), but it was suggested in the review, that this should include the file extension .json.

We could discuss the merits of either approach on this issue.