Twelve-factorise PostgreSQL connection
Twelve-factorise PostgreSQL connection

In order to twelve-factorise Bouncer, we should use DATABASE_URL so that the
database to establish a connection to can be sent by environment variables.
Set a fall-back mechanism to read from the YAML file copied during
deployment if no DATABASE_URL environment variable exists.

alphagov/bouncer,alphagov/bouncer,alphagov/bouncer