Adding a method to get the Directory Tenant Id from the Identity System

Updated eteh README file

Author: shnatara

Identity/AzureStack.Identity.psm1

@@ -1,6 +1,31 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # See LICENSE.txt in the project root for license information.
 
+<#
+.Synopsis
+    Get the Guid of the directory tenant
+.DESCRIPTION
+    This function fetches the OpenID configuration metadata from the identity system and parses the Directory TenantID out of it. 
+    Azure Stack AD FS is configured to be a single tenanted identity system with a TenantID.
+.EXAMPLE
+    Get-DirectoryTenantIdentifier -authority https://login.windows.net/microsoft.onmicrosoft.com
+.EXAMPLE
+    Get-DirectoryTenantIdentifier -authority https://adfs.local.azurestack.external/adfs
+#>
+function Get-DirectoryTenantIdentifier
+{
+    [CmdletBinding()]
+    Param
+    (
+        # Param1 help description
+        [Parameter(Mandatory=$true,
+                    Position=0)]
+        $Authority
+    )
+    
+    return $(Invoke-RestMethod $("{0}/.well-known/openid-configuration" -f $authority.TrimEnd('/'))).issuer.TrimEnd('/').Split('/')[-1]
+}
+
 <#
    .Synopsis
       This function is used to create a Service Principal on teh AD Graph

Identity/README.md

@@ -10,6 +10,16 @@ Then make sure the following modules are imported:
 Import-Module ..\Connect\AzureStack.Connect.psm1
 Import-Module .\AzureStack.Identity.psm1
 ```
+## Getting the directory tenant identifier from the Identity System
+
+This function is used to get the Directory Tenant Guid. This method works for both AAD and AD FS.
+
+```powershell
+$directoryTenantId = Get-DirectoryTenantIdentifier -Authority "<DirectoryTenantUrl>"
+```
+An example of an authority for AAD is `https://login.windows.net/microsoft.onmicrosoft.com`
+and for AD FS is `https://adfs.local.azurestack.global/adfs`.
+
 ## Creating a Service Principal in a disconnected (AD FS) topology
 You can create a Service Principal by executing the following command after importing the Identity Module
 
@@ -20,5 +30,5 @@ $servicePrincipal = New-ADGraphServicePrincipal -DisplayName "<YourServicePrinci
 After the Service Principal is created, you should open your Azure Stack Portal to provide the appropriate level of RBAC to it. You can do this from the Access Control (IAM) tab of any resource. After the RBAC is given, you can login using the service principal as follows:
 
 ```powershell
-Add-AzureRmAccount -EnvironmentName "<AzureStackEnvironmentName>" -ServicePrincipal -CertificateThumbprint $servicePrincipal.Thumbprint -ApplicationId $servicePrincipal.ApplicationId -TenantId "<yourTenantId>"
+Add-AzureRmAccount -EnvironmentName "<AzureStackEnvironmentName>" -ServicePrincipal -CertificateThumbprint $servicePrincipal.Thumbprint -ApplicationId $servicePrincipal.ApplicationId -TenantId $directoryTenantId
 ```