postgresql-on-ubuntu: Deploying on an existing Vnet

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[postgresql-on-ubuntu](https://github.com/Azure/azure-quickstart-templates/tree/master/postgresql-on-ubuntu)

### How do I deploy this inside an existing VNet?

What changes do I have to make it to deploy inside an existing VNet?

Start with the linked postgresql-on-ubuntu template and inspect how its deployment handles networking and VNet parameters. Determine the required changes and prerequisites for using an existing VNet; the issue is done when a reproducible deployment path is documented and validated.

Written by the indexing model from the issue text.