[Bug] Pre-requisite stack launch fails without default VPC

# Please follow the general issue steps first:

- [x] Bug
- [ ] Enhancement
- [ ] Feature Request

### Bug reports:

When trying on instantiate the [pre-requisites stack](https://github.com/aws-samples/serverless-observability-workshop/issues) from the [Create a workspace](https://serverless-observability.workshop.aws/en/020_prerequisites/workspace/workspace.html#region-1) page it fails if the region doesn't have a default subnet. Ideally there would be an optional parameter to specify a subnet.