Added Vagrant configuration cookbook

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | n/a

A new cookbook article was added with an example Vagrant configuration that can be used with the Symfony Standard Edition.
Well, Ubuntu 12.04 is a Long Term Support version (supported until April 2017). So, there will probably a lot of companies using it on their servers which makes it reasonable for their dev teams to also work with this version, doesn't it?