make full curation manifests builder enforce string type for product group ids

Since we couldn't integration test this before now, we've ended up with two different ways to get the product group id into the curation manifest. This change at least ensures that the types are now the same (string rather than int) between full curation submitted from StackStorm and full curation submitted from the WS UI. We'll want to sort this out in a standard manner with 2.8.4.

This has been tested on dev