Improve speed of Test-IsNanoServer function

[mhendric]

Opening a dedicated issue to track the issue @X-Guardian tried to fix in #154. Currently, the Test-IsNanoServer function calls the Get-ComputerInfo cmdlet to check with the computer is running Nano server or not. Get-ComputerInfo can take 40+ seconds to run each time. This function is called for each user and group resource within a Dsc configuration and therefore can make the Dsc application very slow.

Recommended fix:
Replace Get-ComputerInfo with checking the registry value at HKLM:\Software\Microsoft\Windows NT\CurrentVersion\Server\ServerLevels\NanoServer, as documented here: https://msdn.microsoft.com/en-us/library/windows/desktop/hh846315(v=vs.85).aspx