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.
When this cookbook article was initially written, the Vagrant documentation used the `precise32` or `precise64` base box in a number of their examples.  At that time, the Vagrant Cloud service (https://vagrantcloud.com/discover/featured) didn't exist, so these boxes were probably the only ones that could be considered official.  In the original discussion that lead to the creation of this cookbook article, there was a comment about compatibility (https://github.com/symfony/symfony-standard/pull/407#issuecomment-8527747) so the 32 bit version was used.