[COOK-3366] Allow overriding of database_template's cookbook

https://tickets.opscode.com/browse/COOK-3366

I've got a bunch of Rails apps to deploy in very similar ways, so I created a definition to hold the commonality and call the application cookbook; my definition lives in its own cookbook (separate from the small cookbooks that drive it), and has its generic database.yml template (there are a extra parameters I want to configure that the default cookbook doesn't support).

Though I can specify database_template when I invoke application_ruby's rails subresource, it only supports two cookbook locations: the application_ruby cookbook or the driving cookbook.