Azure: some properties of VM should only be set when using spot instances

Running (more or less) the 11.9 Azure release I get:
```
Error: An `eviction_policy` can only be specified when `priority` is set to `Spot`
with module.azure.azurerm_linux_virtual_machine.instances["login1"]
on azure/infrastructure.tf line 56, in resource "azurerm_linux_virtual_machine" "instances":
resource "azurerm_linux_virtual_machine" "instances" {
```