Update IdentityController.cs

[mistermik]

replace client_id with 1950a258-227b-4e31-a9cf-717495945fc2 which is the well-known client ID for Azure PowerShell App, so that we do not have to create a new application registration in AzureAD to leverage the ROPC grant flow. This will make this code to work seamless as, in the documentation is not even mentioned the need to register an application to use the ROPC flow. The use of such AppId is also suggested in others samples as-well, for example: https://docs.microsoft.com/en-us/azure-stack/user/azure-stack-rest-api-use?view=azs-2102