Support using existing VPC

Could we include the option to use existing VPCs?  I'd like it for the Northbound PSC Backend sample to start.  
Looking at the Cloud Foundations module they support it via the the variable [vpc_create](https://github.com/GoogleCloudPlatform/cloud-foundation-fabric/blob/master/modules/net-vpc/main.tf#L37)

I added vpc_create = false to main.tf of the Northbound PSC sample and it worked. 