We used to just try any iface we'd been told about as a
fallback, but that will end up mistakenly using a secondary
network's dns when we really don't have a default connection.
It also messed up our detection of whether we were doing the
lookup on the default or not (we'd get back our secondary net
iface as the default, do the compare and think we were on default).
Remove the lies and let dns fail if we don't have an iface for it.
bug:10132565
Change-Id: I5f0f2abacaaaaf23c5292b20fba9d8dcb6fb10c5
a31ddef36d