I already developed my web app and published it to azure with asp.net core 1.1 and it was working fine until I upgraded my web application to Core 2.0 final
After upgraded to ASP.NET Core 2.0, you need to add following configuration section to all the .csproj files in the solution.
You could create a new ASP.NET Core 2.0 application to check it. The DotNetCliToolReference configuration section is already in the new ASP.NET Core 2.0 application project file but missed in your upgraded application project file.