secrets-tool command for bootstrapping database.

This additional secrets-tool command can be used to run the database
bootsrapping script (`script/database_setup.py`) inside an ATAT docker
container against the Azure database. It sources the necessary keys from
Key Vault.

terraform/secrets-tool/postgres-user.yaml

@@ -0,0 +1,4 @@
+---
+- PGPASSWORD:
+    type: 'password'
+    length: 30