Fix System.DirectoryServices configurations and generate PNS assembly correctly for UAP (dotnet/corefx#24878)
* Fix System.DirectoryServices configurations and generate PNS assembly correctly
* Fix CI adding UapVNext as a target framework for the pkg
* Fix configurations to be PNSE when targeting netstandard or any non-windows configuration
* Disable test that is using DirectoryServices and is getting PNSE
Commit migrated from https://github.com/dotnet/corefx/commit/
3be15ca3d519fafc149c08b20727f88cb0f76e6a