azure_rm_aks Add parameter vm-set-type

Hi there,

the Ansible support for creating AKS clusters comes in very handy, thanks for that! I have one request though: I woud like to use the cluster autoscaler with AKS. However, I cannot enable that because the for doing so the cluster needs to be created with support for _virtual mchine scale sets_, see https://docs.microsoft.com/en-us/azure/aks/cluster-autoscaler for details.

Essentially, I think it would suffice to add support for the parameter `vm-set-type VirtualMachineScaleSets` as being passed into `az aks create`. Or perhaps even the option `enable-cluster-autoscaler` to enable it right away.

Thanks in advance and kind regards,
Philipp