Database users

When cooking server with mysql+rails roles (on master branch, commit 816f012), mysql user for the app database is not created. Thus, you cannot deploy an app to the server due to the error:

    Mysql2::Error: Access denied for user 'app_user'@'localhost' (using password: YES)

The rails::database was removed from the rails recipe, but not from the rails_passenger one. With this pull request I suggest remove the recipe from the passenger role as well, and add the recipe to the sample_node.json example.

The request also includes other fixes:

* Fixed absent bundler recipe for unicorn role (issue #58)
* Bluepill workaround for new versions of Ubuntu (at least for Vagrant, issue #24)
* Updated the "Open an issue" link in the README.md to match current repo location