Updated terraform-provider-google min version to 1.12.0
Updated terraform-provider-google min version to 1.12.0

enable_flow_logs field to subnetwork resource was introduced in
terraform-provider-google version 1.12.0. Used at
https://github.com/rbramwell/terraform-google-network/blob/master/main.tf#L37
. See
https://github.com/terraform-providers/terraform-provider-google/blob/master/CHANGELOG.md#1120-may-04-2018

terraform-google-modules/terraform-google-network,terraform-google-modules/terraform-google-network