Enable use of vagrant-omnibus plugin in generated vagrant files

Opscode no longer provide base boxes with chef preinstalled and instead recommend the use of the `vagrant-omnibus` plugin to specify the chef version to install.

These changes allow a chef version to be specified in the berkshelf config so that it can be set by default in generated cookbooks.

As a bonus I have added a cookbook and vagrant file to the project in a separate commit so that vagrant can be used for development of berkshelf.