Module installation fails when default VPC network is a Legacy network

[dboshardy]

When using this module as described (setting source = GoogleCloudPlatform/managed-instance-group/google), terraform apply fails with the following:

* google_compute_instance_template.default: Error creating instance template: googleapi: Error 404: The resource 'projects/cloud9-dev/regions/us-central1/subnetworks/default' was not found, notFound```

This is a result of the `default` VPC network in GCP being a Legacy network with no subnetwork.