AzureEnvironment.cs has reference to invalid domain name discover.azure.com

When going through the authentication flow with Azure there are several attempts to hit discover.azure.com.
The domain name is not documented anywhere and also always results in a name resolution failure.
This has been criticized by several enterprise security admins recently because they do not want to enable a URL that does not exist.
It is only used to handle an empty Metadata endpoint; we should change it to an endpoint that works or handle the nullOrEmpty case differently.