staging: Add clearly-defined postgresql instance
This creates an empty db instance on the staging db1 instance.
The users created alongside are:
- user guest with read-only access
- user clearly-defined with write access
Related to T2865
Test Plan
bin/octocatalog-diff demonstrates the new db instance creation.
vagrant up staging-db1 happily creates the new db
Migrated from D4745 (view on Phabricator)