(NC-137) Return group w/uuid from API.

This involves putting the resource returned by the storage's create-*
function in the context, rather than the resource created from the
user's request, since only the former will have the UUID (since it is
added by the create-* function itself). As a consequence, all of the
Storage protocol's create-* functions must return the resource as
inserted, as well as mock Storage implementations for tests.

Add acceptance test for getting the group by its UUID.