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

[mkostersitz]

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.