Move Vagrant's dmg_package source & checksum to attributes file

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
Merge branch 'attribute-customizable-vagrant-version' into develop