Support GKE on Windows as well as we're likely to be able to
Support GKE on Windows as well as we're likely to be able to

Fixes #431

Note that this doesn't get full details - it can't detect the
container name, because it doesn't have mount info. However, it
looks like it gets everything else (somewhat remarkably).

googleapis/gax-dotnet,jskeet/gax-dotnet,jskeet/gax-dotnet,googleapis/gax-dotnet