Update "Effective Core Count" calculations for VMSS

In some places (e.g. https://github.com/Azure/azure-functions-host/blob/638e46d91b7b6ba61a4abc1a46d1ddb39ee467ec/src/WebJobs.Script.WebHost/Program.cs#L119) we calculate the number of effective cores based on SKU restrictions. We need to take VMSS into account in those places.

We should have a centralized method that does this computation.

@brettsam @safihamid FYI

Start in src/WebJobs.Script.WebHost/Program.cs around line 119 and search for the other effective-core calculations based on SKU restrictions. Trace how VMSS information is available at each call site, then centralize the computation so all identified locations account for VMSS; verify the affected host behavior with the relevant existing tests.

Written by the indexing model from the issue text.