Move Vagrant's dmg_package source & checksum to attributes file

On my local machine, I wanted to install a newer version of Vagrant, so I decided to move the dmg properties for the `sprout-osx-apps::vagrant` recipe to default attributes.  Then I can just add the version I want in my soloistrc.

This pull request doesn't change the behavior of the `sprout-osx-apps::vagrant` recipe, without custom node attributes, it still defaults to version 1.0.5 of Vagrant as before.

Notes:
---------

  By default, the recipe will use version 1.0.5 of Vagrant.
  To use a custom version of vagrant, put something like this in your
  soloistrc:

    node_attributes:
      sprout:
        vagrant:
          dmg:
            source: http://files.vagrantup.com/packages/7e400d00a3c5a0fdf2809c8b5001a035415a607b/Vagrant-1.2.2.dmg
            checksum: 1581552841e076043308f330a5b1130b455c604846116c54b5330bb17240c7ee