Use Open Directory for VPN authentication on OS X Server

Active Directory credentials can't be used to authenticate VPN connections on OS X Server (Yosemite).

When setting up VPN services in OS X Server (Yosemite) use Open Directory to establish credentials for VPN users. If a client tries to authenticate to your VPN connection using Active Directory credentials, VPN authentication will fail.

After a client establishes a VPN connection, the client can then use Active Directory to authenticate other services available on your server.