CHEF-3711: Knife Windows gem fails bootstrap during download step on Win2k12

Modifies the Windows bootstrap erb template that generates the batch file used to bootstrap Windows nodes. The modification adds a fallback to PowerShell script if msi download via vbscript fails, which is currently the case on Win2k12 (see http://tickets.opscode.com/browse/CHEF-3711). 

Functional tests are also added to verify the template rendering, the successful download case on all Windows platforms, and the failure case on Win2k12. The tests have been run on MacOS, Win2k8R2, and Win2k12.