Make increasing HPC/GPU node count simpler

If a CX wants to increase their VMSS size from 100 to say, 300, they have to set both the MaxCount parameter and the MaxScaleSetSize parameter. We could use an expression for MaxScaleSetSize of :=max(MaxCount, 100) as a default. It is very unusual, as of azslurm 3.x, that we use more than a single VMSS in a partition, so this would make this simpler.