Enable building our repo's without requiring Visual Studio installed

@weshaggard commented on [Thu Dec 07 2017](https://github.com/dotnet/core-eng/issues/2239)

In order to make our builds more reliable and maintainable long term we need to be able to bootstrap our toolsets as part of our build initialization and stop relying on globally installed tools. This includes things like CMake and Visual Studio but for this issue I want to focus on what it would take to break our dependency on requiring Visual Studio. Note that I think we should support using Visual Studio if it is present but we should be able to build without it and allow folks to isolate themselves and build without it even if it is installed.

There are a few things I'm aware of that need to be figured out in order to break our VS dependency:
1) We need msbuild along with the managed tools and targets.
- We should be able to use what comes with the SDK to handle the managed builds.
2) We need C++ compiler as well as the headers and libs.
- We should be able to look https://aka.ms/dailyMSVC and switch to downloading their nuget packages.
3) We need the Windows SDK to be able to build things targeting UWP.
- We need to investigate our options. Assuming it is just some libs and header perhaps we create our own custom package to provide this.

cc @shawnro @dleeapho