Extend NMDefaultRouteManager to track NMVpnConnection beside
NMDevice. That way, all default routes are managed by
NMDefaultRouteManager.

For VPN connections the manager also tracks connections that are
set never_default. That is useful because NMPolicy still uses VPNs
without default route to setup DNS. Hence, NMDefaultRouteManager
trackes those connections to have the relative priority of the
devices.
Interestingly, that means that for VPNs that are ipv4.never-default,
ipv4.route-metric still has an effect in determining relative priorities
for DNS configuration.

This commit only adds the parts to track the default route. NMPolicy
still sets the route as before.

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