The previous commit made NM enforce the default route on interfaces for
which NM manages a default route.

For interfaces that are configured never-default, NM will now pick up
any externally configured default route, as if it was managed by NM.
This is important, because NMDefaultRouteManager needs a notion of which
is the best device. Without this change, it was agnostic to default routes
on managed, never-default interfaces.

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