Win2016 images - fix TLS protocol issue to allow installation of NuGet

[urisimchoni]

https://rnelson0.com/2018/05/17/powershell-in-a-post-tls1-1-world/

It looks like the Win2016 image has a powershell version whose default TLS settings are no longer sufficient for downloading stuff from Microsoft. Maybe this can also be fixed by downloading a newer installer ISO version, but I couldn't figure out where to get the most up-to-date image from.