Now that both VPN and devices are managed (and ordered) by
NMDefaultRouteManager, refactor get_best_config() to use the
priority accordingly.

Before, we would first iterate over all VPN connections and
returning the best one. Only if no suitable VPN connection
was found, a best device would be returned.
Modify get_best_config() to treat VPN and device the same and
return the best one based on the route metric.

With this change, get_best_config() gives consistent results
together with get_best_device(). Also, you can configure
that a device gets a higher priority then a VPN.

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