Limit Supermarket Postgres user privileges

This alters the postgres recipe so the supermarket user only has login
permissions and is granted access to the database created for
Supermarket. Since SUPERUSER permissions are required to enable
extensions this moves extension creation into the postgres recipe and
removes it from the application schema/migrations.
Application recipe should generate a database.yml

The application recipe now generates a database.yml for use in
production on the fly based on the recipes attributes for postgres
database, username and pool size.