Create database with separate script.

Creating the ATAT database requires a separate connection to one of the
default Postgres databases, like `postgres`. This updates the scripts
and secrets-tool command to handle creating the database. It also
removes database creation from Terraform and updates the documentation.

terraform/modules/postgres/outputs.tf

@@ -1,3 +0,0 @@
-output "db_name" {
-  value = azurerm_postgresql_database.db.name
-}