Fix windows 2012 product key definition for unattended installation
otherwise you get a "Windows cannot read the <Product Key> setting from the unattend answer file" error
@see http://technet.microsoft.com/en-US/library/cc721925(v=ws.10).aspx
Merge pull request #994 from tnguyen1/win2012-productkey

Fix windows 2012 product key definition for unattended installation