Update Packer SP Creation PowerShell Commands

Updates to the commands to create the Azure AD service principal to be used for authentication by Packer. Due to changes in the return of New-AzADServicePrincipal there is no longer a Secret value returned, the SP credentials must be set by using New-AzADSpCredential

Author: cocallaw

articles/virtual-machines/windows/build-image-with-packer.md

@@ -42,15 +42,14 @@ Create a service principal with [New-AzADServicePrincipal](/powershell/module/az
 
 ```azurepowershell
 $sp = New-AzADServicePrincipal -DisplayName "PackerSP$(Get-Random)"
-$BSTR = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($sp.Secret)
-$plainPassword = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR)
+$plainPassword = (New-AzADSpCredential -ObjectId $sp.Id).SecretText
 ```
 
 Then output the password and application ID.
 
 ```powershell
 $plainPassword
-$sp.ApplicationId
+$sp.AppId
 ```
 
 
@@ -68,7 +67,7 @@ Create a file named *windows.json* and paste the following content. Enter your o
 
 | Parameter                           | Where to obtain |
 |-------------------------------------|----------------------------------------------------|
-| *client_id*                         | View service principal ID with `$sp.applicationId` |
+| *client_id*                         | View service principal ID with `$sp.AppId` |
 | *client_secret*                     | View the auto-generated password with `$plainPassword` |
 | *tenant_id*                         | Output from `$sub.TenantId` command |
 | *subscription_id*                   | Output from `$sub.SubscriptionId` command |