Allow using a separate database server

[javierm]

Using one server for the application and a different one for the database seems to be a common pattern.

We define a database_hostname which is used in the generated database.yml file. It would be great if the tasks creating the database, its users and its extensions were executed on the server defined in database_hostname.

Alternatively, we could provide some kind of condition to easily skip these steps so they don't run when we're using a separate database server.