Add gateway as direct route for interface

Signed-off-by: Guilhem Lettron <guilhem.lettron@gmail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Some tweaks to direct route to gateway

The previous patch added code to add a static route prior to adding the
gateway to the interface.

This commit simply changes the logic so that this is only done on
failure to add the gateway.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>