#28 - adds 'box' configuration option for vagrant VMs

Changes `vm.box` to be a generic name so that multiple vagrant nodes can share the same basebox. `box` value resolution is as follows:

  * `box` from the node's `vagrant` configuration hash
  * `box` from the default vagrant configuration
  * the tail of the box_url (e.g. `centos-6.4` if box_url is `https://opscode.s3.amazonaws.com/centos-6.4.box`)