Support gateway in NetworkSelectionElement

Changes config JSON from gateway to default-route, adds Readme, fixes lint

Co-authored-by: dougbtv <dosmith@redhat.com>

netutils/netutils.go

@@ -0,0 +1,62 @@
+// Copyright (c) 2019 Multus Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+package netutils
+
+import (
+	"github.com/containernetworking/cni/pkg/skel"
+	cnitypes "github.com/containernetworking/cni/pkg/types"
+	"github.com/containernetworking/cni/pkg/types/current"
+	"github.com/containernetworking/plugins/pkg/ns"
+
+	"github.com/intel/multus-cni/logging"
+
+	"github.com/vishvananda/netlink"
+)
+
+// DeleteDefaultGW removes the default gateway from marked interfaces.
+func DeleteDefaultGW(args *skel.CmdArgs, ifName string, res *cnitypes.Result) (*current.Result, error) {
+	logging.Debugf("XXX: DeleteDefaultGW: %s", args.Netns)
+	result, err := current.NewResultFromResult(*res)
+	if err != nil {
+		return nil, logging.Errorf("XXX: %v", err)
+	}
+
+	netns, err := ns.GetNS(args.Netns)
+	if err != nil {
+		return nil, logging.Errorf("XXX: %v", err)
+	}
+	defer netns.Close()
+
+	err = netns.Do(func(_ ns.NetNS) error {
+		var err error
+		link, _ := netlink.LinkByName(ifName)
+		routes, _ := netlink.RouteList(link, netlink.FAMILY_ALL)
+		for _, nlroute := range routes {
+			if nlroute.Dst == nil {
+				err = netlink.RouteDel(&nlroute)
+			}
+		}
+		return err
+	})
+	var newRoutes []*cnitypes.Route
+	for _, route := range result.Routes {
+		if mask, _ := route.Dst.Mask.Size(); mask != 0 {
+			newRoutes = append(newRoutes, route)
+		}
+	}
+	result.Routes = newRoutes
+	return result, err
+}