DSC spec platform detection via WMI

Specs for DSC should only run if you have DSC support. Use WMI to check for the presence of MSFT_DSCLocalConfigurationManager in root/microsoft/windows/desiredstateconfiguration.
Well, this is just a helper for specs, so wasn't planning on logging. I expect this case to occur on systems without powershell 4, so it's not really an error case.