Edits /etc/network/interface on network node

This patch edits the configuration of the interface file
to add configuration of the external bridge br-ex for the
machine to access the external network.

Change-Id: Ib14ada742ee03556ffa0cd1b1947b280574a2785
Closes-bug: 1371060
Merge "Edits /etc/network/interface on network node"