Add 'priority' property for VM scale set

Purpose is to support low-priority VMs, which should partially
address #1249.

This is simply adding a 'priority' parameter that can be set
to Low or Regular (=default).

The evictionPolicy is not supported yet as it was introduced
in a more recent version of the compute API (2018-04-01).

Cf https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-use-low-priority

azurerm/resource_arm_virtual_machine_scale_set.go

@@ -104,6 +104,11 @@ func resourceArmVirtualMachineScaleSet() *schema.Resource {
 				ForceNew: true,
 			},
 
+			"priority": {
+				Type:     schema.TypeString,
+				Optional: true,
+			},
+
 			"os_profile": {
 				Type:     schema.TypeList,
 				Required: true,
@@ -640,6 +645,7 @@ func resourceArmVirtualMachineScaleSetCreate(d *schema.ResourceData, meta interf
 	updatePolicy := d.Get("upgrade_policy_mode").(string)
 	overprovision := d.Get("overprovision").(bool)
 	singlePlacementGroup := d.Get("single_placement_group").(bool)
+	priority := d.Get("priority").(string)
 
 	scaleSetProps := compute.VirtualMachineScaleSetProperties{
 		UpgradePolicy: &compute.UpgradePolicy{
@@ -650,6 +656,7 @@ func resourceArmVirtualMachineScaleSetCreate(d *schema.ResourceData, meta interf
 			StorageProfile:   &storageProfile,
 			OsProfile:        osProfile,
 			ExtensionProfile: extensions,
+			Priority:         compute.VirtualMachinePriorityTypes(priority),
 		},
 		Overprovision:        &overprovision,
 		SinglePlacementGroup: &singlePlacementGroup,
@@ -746,6 +753,7 @@ func resourceArmVirtualMachineScaleSetRead(d *schema.ResourceData, meta interfac
 	d.Set("upgrade_policy_mode", properties.UpgradePolicy.Mode)
 	d.Set("overprovision", properties.Overprovision)
 	d.Set("single_placement_group", properties.SinglePlacementGroup)
+	d.Set("priority", properties.VirtualMachineProfile.Priority)
 
 	osProfile, err := flattenAzureRMVirtualMachineScaleSetOsProfile(d, properties.VirtualMachineProfile.OsProfile)
 	if err != nil {

azurerm/resource_arm_virtual_machine_scale_set_test.go

@@ -437,6 +437,25 @@ func TestAccAzureRMVirtualMachineScaleSet_overprovision(t *testing.T) {
 	})
 }
 
+func TestAccAzureRMVirtualMachineScaleSet_priority(t *testing.T) {
+	ri := acctest.RandInt()
+	config := testAccAzureRMVirtualMachineScaleSetPriorityTemplate(ri, testLocation())
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testCheckAzureRMVirtualMachineScaleSetDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: config,
+				Check: resource.ComposeTestCheckFunc(
+					testCheckAzureRMVirtualMachineScaleSetExists("azurerm_virtual_machine_scale_set.test"),
+					testCheckAzureRMVirtualMachineScaleSetPriority("azurerm_virtual_machine_scale_set.test", "Low"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAzureRMVirtualMachineScaleSet_MSI(t *testing.T) {
 	resourceName := "azurerm_virtual_machine_scale_set.test"
 	ri := acctest.RandInt()
@@ -812,6 +831,21 @@ func testCheckAzureRMVirtualMachineScaleSetOverprovision(name string) resource.T
 	}
 }
 
+func testCheckAzureRMVirtualMachineScaleSetPriority(name string, expectedPriority string) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		resp, err := testGetAzureRMVirtualMachineScaleSet(s, name)
+		if err != nil {
+			return err
+		}
+
+		if resp.VirtualMachineProfile.Priority != compute.VirtualMachinePriorityTypes(expectedPriority) {
+			return fmt.Errorf("Bad: Priority should have been %s for scale set %v", expectedPriority, name)
+		}
+
+		return nil
+	}
+}
+
 func testCheckAzureRMVirtualMachineScaleSetSinglePlacementGroup(name string, expectedSinglePlacementGroup bool) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		resp, err := testGetAzureRMVirtualMachineScaleSet(s, name)
@@ -820,7 +854,7 @@ func testCheckAzureRMVirtualMachineScaleSetSinglePlacementGroup(name string, exp
 		}
 
 		if *resp.SinglePlacementGroup != expectedSinglePlacementGroup {
-			return fmt.Errorf("Bad: Overprovision should have been %t for scale set %v", expectedSinglePlacementGroup, name)
+			return fmt.Errorf("Bad: SinglePlacementGroup should have been %t for scale set %v", expectedSinglePlacementGroup, name)
 		}
 
 		return nil
@@ -2577,6 +2611,90 @@ resource "azurerm_virtual_machine_scale_set" "test" {
 `, rInt, location, rInt, rInt, rInt, rInt, rInt)
 }
 
+func testAccAzureRMVirtualMachineScaleSetPriorityTemplate(rInt int, location string) string {
+	return fmt.Sprintf(`
+resource "azurerm_resource_group" "test" {
+  name     = "acctestrg-%d"
+  location = "%s"
+}
+
+resource "azurerm_virtual_network" "test" {
+  name                = "acctvn-%d"
+  address_space       = ["10.0.0.0/16"]
+  location            = "${azurerm_resource_group.test.location}"
+  resource_group_name = "${azurerm_resource_group.test.name}"
+}
+
+resource "azurerm_subnet" "test" {
+  name                 = "acctsub-%d"
+  resource_group_name  = "${azurerm_resource_group.test.name}"
+  virtual_network_name = "${azurerm_virtual_network.test.name}"
+  address_prefix       = "10.0.2.0/24"
+}
+
+resource "azurerm_storage_account" "test" {
+  name                     = "accsa%d"
+  resource_group_name      = "${azurerm_resource_group.test.name}"
+  location                 = "${azurerm_resource_group.test.location}"
+  account_tier             = "Standard"
+  account_replication_type = "LRS"
+}
+
+resource "azurerm_storage_container" "test" {
+  name                  = "vhds"
+  resource_group_name   = "${azurerm_resource_group.test.name}"
+  storage_account_name  = "${azurerm_storage_account.test.name}"
+  container_access_type = "private"
+}
+
+resource "azurerm_virtual_machine_scale_set" "test" {
+  name                = "acctvmss-%d"
+  location            = "${azurerm_resource_group.test.location}"
+  resource_group_name = "${azurerm_resource_group.test.name}"
+  upgrade_policy_mode = "Manual"
+  overprovision       = false
+  priority            = "Low"
+
+  sku {
+    name     = "Standard_D1_v2"
+    tier     = "Standard"
+    capacity = 1
+  }
+
+  os_profile {
+    computer_name_prefix = "testvm-%d"
+    admin_username       = "myadmin"
+    admin_password       = "Passwword1234"
+  }
+
+  network_profile {
+    name    = "TestNetworkProfile"
+    primary = true
+
+    ip_configuration {
+      name      = "TestIPConfiguration"
+      subnet_id = "${azurerm_subnet.test.id}"
+    }
+  }
+
+  storage_profile_os_disk {
+    name           = "os-disk"
+    caching        = "ReadWrite"
+    create_option  = "FromImage"
+    vhd_containers = ["${azurerm_storage_account.test.primary_blob_endpoint}${azurerm_storage_container.test.name}"]
+  }
+
+  storage_profile_image_reference {
+    publisher = "Canonical"
+    offer     = "UbuntuServer"
+    sku       = "16.04-LTS"
+    version   = "latest"
+  }
+}
+
+`, rInt, location, rInt, rInt, rInt, rInt, rInt)
+}
+
 func testAccAzureRMVirtualMachineScaleSetMSITemplate(rInt int, location string) string {
 	return fmt.Sprintf(`
 resource "azurerm_resource_group" "test" {