For bearers using STATIC or DHCP IP method, the modem itself is the one
negotiating authentication with the network. The new `allowed-auth' property
allows users to specify which authentication method(s) are allowed to be used.
See the following NetworkManager commit for more reference:
commit 34aef8aaaa09b7473b9496aa49e550bd2def03f8
Author: Andrew Bird <ajb@spheresystems.co.uk>
Date: Thu Mar 15 16:19:43 2012 -0500
ebd792d2aa