Allow the existing certificate in the App Registration to be used.

In development scenarios where we have to redeploy Nerdio for training/demo purposes, an error will occur when running the setup script for the App Service. The issue here is that the ‘custom certificated in App registration’ was created by the previous Nerdio deployment. So a fix here would be great so if we had to redeploy for any reason, we do not have to delete the old app registration.