addons: addressvirtual: new policy (bool): addressvirtual_with_route_metric (default...

addons: addressvirtual: new policy (bool): addressvirtual_with_route_metric (default: yes)

Ifupdown2 is now setting a default metric on macvlan ips. This policy will let
users disable this new default behavior. addressvirtual_with_route_metric is
boolean policy variable.

  addressvirtual_with_route_metric: yes|no|on|off|1|0 (default to yes)

$ cat /var/lib/ifupdown2/policy.d/addressvirtual.json
{
    "addressvirtual": {
        "module_globals": {
            "addressvirtual_with_route_metric": "no"
        }
    }
}

Reviewed-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>