Add Support for Larger Size VM Pools

[alzimmermsft]

At the time of writing this issue, the VMs used in CI and testing are 4 vCPU machines. In most situations, the size of these machines is sufficient for good CI times, as there can be many situations where CI and testing isn't capable of scaling up to large VM sizes. But for some CI jobs and test jobs they scale very well for a larger machine size.

We should look into offering support for a scale up scenario, likely with machines that are 8 vCPUs.