net/mwan3: add online_metric for local_source none

net/mwan3: add online_metric for local_source none

If we set the option "local_source" in the globals mwan3 section to "none",
traffic generated by the router it self will always use the default route from
the wan interface with the lowest metric. If this interface is down
the router traffic still uses the connection with the lowest metric but
this is disconnected. Load balancing and failover from the lan site is
still possible. Only router generated traffic is not load balanced and
could not use failover.

To solve this issue with router initiated traffic add the additional
option "online_metric" to the mwan3 interface section.

If the interface is connected then this lower "online metric" is set in the
default routing table.

With this change we have at least a failover with router initiated
traffic.

Signed-off-by: Florian Eckert <redacted>