External interface shows as up fake

On the network topology router-mouse-over-balloon, the list of
interfaces contains one labeled "fake<some_id>" which is a placeholder
for the external interface.

This interface should be labeled with a much less confusing tag.
"extern" or "gateway" seems much better.

Closes-Bug: #1273774

Change-Id: I873f80244757f573778abcac746c32c26732c8a9
Merge "External interface shows as up fake"