Up to now, NMPolicy would iterate over all devices to find the "best"
device and assign the default route to that device.

A better approach is to add a default route to *all* devices that
are never-default=no. The relative priority is choosen according to
the route metrics.

If two devices receive the same metric, we want to prefer the device
that activates first. That way, the default route sticks to the same
device until a better device activates or the device deactivates.
Hence, the order of activation is imporant in this case (as it is
already now).

Also, if several devices have identical metrics, increment their
metrics so that every metric is unique.
This makes the routing deterministic according to what we choose as best
device.

A special case is assumed devices. In this case we cannot adjust the metric
in face of equal metrics.

Add a new singleton class NMDefaultRouteManager that has a list of all
devices and their default routes. The manager will order the devices by
their priority and configure the routes using platform.

Also update the metric for VPN connections. Later we will track VPN
routes also via NMDefaultRouteManager. For now, fix the VPN metric because
otherwise VPNs would always get metric 1024 (which is usually much larger then the
device metrics).

https://bugzilla.gnome.org/show_bug.cgi?id=735512



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