Fix Shared VPC use case

Following the Terraform documentation of compute_instance_template resource (https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/compute_instance_template#subnetwork_project), network, subnetwork and subnetwork_project are optional, but if we try to specify all of them, when the resource is created, it tries to find the network on the service project instead on the host project, so resource creation fails.

This release has a workaround to avoid this: if we set subnet_name variable, we force network_name to be null.