Implement a default database URL

[ekohl]

This implements a default database URL, which is postgresql:// for production and postgresql:///foreman-$ENV otherwise. That means using a unix socket to connect to, which implies the username and doesn't need any password. When no database name is given (like in production) PostgreSQL assumes one that matches the username.

By doing this, there is no need for a config/database.yml file which simplifies setup.

I'm not sure if this is a good idea or not. It'll still be needed for remote DBs and in case you need to override the pool size. It does make development and testing easier.