provider/azurerm: Update `azurerm_virtual_machine` failed on

adminPassword

The Azure API never returns the AdminPAssword (as is correct) from the
Read API call. Therefore on Create, we do not set the AdminPassword of
the vm as part of the state. The Same func is used for Create & Update,
therefore when we changed anything on the VM, we were getting the
following error:

```
statusCode:Conflict
serviceRequestId:f498a6c8-6e7a-420f-9788-400f18078921
statusMessage:{"error":{"code":"PropertyChangeNotAllowed","target":"adminPassword","message":"Changing property 'adminPassword' is not allowed."}}
```

To fix this, we need to excldue the AdminPassword from the Update func
if it is empty

builtin/providers/azurerm/resource_arm_virtual_machine.go

@@ -814,7 +814,10 @@ func expandAzureRmVirtualMachineOsProfile(d *schema.ResourceData) (*compute.OSPr
 
 	profile := &compute.OSProfile{
 		AdminUsername: &adminUsername,
-		AdminPassword: &adminPassword,
+	}
+
+	if adminPassword != "" {
+		profile.AdminPassword = &adminPassword
 	}
 
 	if _, ok := d.GetOk("os_profile_windows_config"); ok {

builtin/providers/azurerm/resource_arm_virtual_machine_test.go

@@ -28,6 +28,37 @@ func TestAccAzureRMVirtualMachine_basicLinuxMachine(t *testing.T) {
 	})
 }
 
+//This is a regression test around https://github.com/hashicorp/terraform/issues/6517
+//Because we use CreateOrUpdate, we were sending an empty password on update requests
+func TestAccAzureRMVirtualMachine_updateMachineSize(t *testing.T) {
+	ri := acctest.RandInt()
+	preConfig := fmt.Sprintf(testAccAzureRMVirtualMachine_basicLinuxMachine, ri, ri, ri, ri, ri, ri, ri)
+	postConfig := fmt.Sprintf(testAccAzureRMVirtualMachine_updatedLinuxMachine, ri, ri, ri, ri, ri, ri, ri)
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testCheckAzureRMVirtualMachineDestroy,
+		Steps: []resource.TestStep{
+			resource.TestStep{
+				Config: preConfig,
+				Check: resource.ComposeTestCheckFunc(
+					testCheckAzureRMVirtualMachineExists("azurerm_virtual_machine.test"),
+					resource.TestCheckResourceAttr(
+						"azurerm_virtual_machine.test", "vm_size", "Standard_A0"),
+				),
+			},
+			resource.TestStep{
+				Config: postConfig,
+				Check: resource.ComposeTestCheckFunc(
+					testCheckAzureRMVirtualMachineExists("azurerm_virtual_machine.test"),
+					resource.TestCheckResourceAttr(
+						"azurerm_virtual_machine.test", "vm_size", "Standard_A1"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAzureRMVirtualMachine_basicWindowsMachine(t *testing.T) {
 	ri := acctest.RandInt()
 	config := fmt.Sprintf(testAccAzureRMVirtualMachine_basicWindowsMachine, ri, ri, ri, ri, ri, ri)
@@ -183,6 +214,89 @@ resource "azurerm_virtual_machine" "test" {
 }
 `
 
+var testAccAzureRMVirtualMachine_updatedLinuxMachine = `
+resource "azurerm_resource_group" "test" {
+    name = "acctestrg-%d"
+    location = "West US"
+}
+
+resource "azurerm_virtual_network" "test" {
+    name = "acctvn-%d"
+    address_space = ["10.0.0.0/16"]
+    location = "West US"
+    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 = "${azurerm_virtual_network.test.name}"
+    address_prefix = "10.0.2.0/24"
+}
+
+resource "azurerm_network_interface" "test" {
+    name = "acctni-%d"
+    location = "West US"
+    resource_group_name = "${azurerm_resource_group.test.name}"
+
+    ip_configuration {
+    	name = "testconfiguration1"
+    	subnet_id = "${azurerm_subnet.test.id}"
+    	private_ip_address_allocation = "dynamic"
+    }
+}
+
+resource "azurerm_storage_account" "test" {
+    name = "accsa%d"
+    resource_group_name = "${azurerm_resource_group.test.name}"
+    location = "westus"
+    account_type = "Standard_LRS"
+
+    tags {
+        environment = "staging"
+    }
+}
+
+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" "test" {
+    name = "acctvm-%d"
+    location = "West US"
+    resource_group_name = "${azurerm_resource_group.test.name}"
+    network_interface_ids = ["${azurerm_network_interface.test.id}"]
+    vm_size = "Standard_A1"
+
+    storage_image_reference {
+	publisher = "Canonical"
+	offer = "UbuntuServer"
+	sku = "14.04.2-LTS"
+	version = "latest"
+    }
+
+    storage_os_disk {
+        name = "myosdisk1"
+        vhd_uri = "${azurerm_storage_account.test.primary_blob_endpoint}${azurerm_storage_container.test.name}/myosdisk1.vhd"
+        caching = "ReadWrite"
+        create_option = "FromImage"
+    }
+
+    os_profile {
+	computer_name = "hostname%d"
+	admin_username = "testadmin"
+	admin_password = "Password1234!"
+    }
+
+    os_profile_linux_config {
+	disable_password_authentication = false
+    }
+}
+`
+
 var testAccAzureRMVirtualMachine_basicWindowsMachine = `
 resource "azurerm_resource_group" "test" {
     name = "acctestrg-%d"