Add --ultra-ssd-enabled true flag to worker VM provisioning to enable UltraSSD persistent volumes

Many clients require the use of UltraSSD disks for performance reasons on ARO. The request here is to enable UltraSSD support on the CoreOS worker VMs during provisioning. This service already exists in AKS and clients are opting to use AKS instead of ARO for this reason (https://docs.microsoft.com/en-us/azure/aks/use-ultra-disks).  