Add support for vagrant-windows

The bare minimum configuration necessary to make config_builder
compatible with vagrant-windows is to allow a specification of "guest".
E.g.

    - name: "server2008r2a"
      box: "windows-server-2008-r2-x64-v2"
      guest: windows

This commit also allows specification of guest in a role.
Merge pull request #12 from reidmv/windows-support

Add support for vagrant-windows