Avoid unneeded rotations of Regular priority virtual machine scale sets

* Azure only allows `eviction_policy` to be set for Low priority VMs.
Supporting Low priority VMs meant when Regular VMs were used, each
`terraform apply` rolled workers, to set eviction_policy to null.
* Terraform v0.12 nullable variables fix the issue and plan does not
produce a diff

azure/container-linux/kubernetes/workers/workers.tf

@@ -68,8 +68,9 @@ resource "azurerm_virtual_machine_scale_set" "workers" {
 
   # lifecycle
   upgrade_policy_mode = "Manual"
+  # eviction policy may only be set when priority is Low
   priority            = var.priority
-  eviction_policy     = "Delete"
+  eviction_policy     = var.priority == "Low" ? "Delete" : null
 }
 
 # Scale up or down to maintain desired number, tolerating deallocations.