Add authentication to "MVNO NL" APN's

Future services will require authentication on both "MVNO NL" APNs. This
enhancement will be backward compatible for current users of standard APN
services. Authentication is done via generic username+password and PAP.

Change-Id: Ic772a46b1f443600a786df59a593e21b64f7f1df
Signed-off-by: Arjan van der Oest <arjan.vanderoest@voiceworks.com>