Before, we would only track a device in NMDefaultRouteManager
if it had a default route. Otherwise the entry for the device
was removed.

That was wrong, because having no entry meant that the interface
is assumed and hence we would not touch the interface. Instead we must
esplicitly track devices without default route to know when an interface
has no default route.

Signed-off-by: Thomas Haller <thaller@redhat.com>