Update C# Samples to be MSBuild 15 style

[matthchr]

• Update C# csproj format to the new and much improved msbuild 15 style. This means that users must use Visual Studio 2017 now.
• Move to use the newly renamed Azure Batch C# SDK (Microsoft.Azure.Batch as opposed to the old Azure.Batch)
• Use the new netstandard configuration namespace to make converting the projects to netcore for customers easier.

[matthchr]

I manually tested all of the samples and they still work.

The only thing I didn't test was the "ApplicationInsights" portion of the AppInsights sample. The tasks run but I don't have an appinsights account set up to test that part.

[matthchr]

@dlepow - can you look at the stuff in the ArticleProjects folder and make the corresponding article updates if you feel anything meets the bar there?

I basically didn't change any logic, but each article has:

1. A few minor indentation fixes.
2. A move to OS Family 5 if it wasn't already using it for the pool.
3. A move to standard_d1_v2 from small for the VM size.
4. A small change in how the configuration is loaded for all article projects except the GettingStarted one where we don't use that configuration mechanism.

As an aside, do we want to keep the GettingStarted sample now that we also have quickstarts?