Replace usage of WithNetNSUnsafe in plugin/net/watcher.go with safer version

Unsafe version was in ConfigureARPforVeths; replacement is via `weaveutil configure-arp`

plugin/net/watcher.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 
 	weaveapi "github.com/weaveworks/weave/api"
-	"github.com/weaveworks/weave/common"
 	"github.com/weaveworks/weave/common/docker"
 	weavenet "github.com/weaveworks/weave/net"
 )
@@ -46,7 +45,7 @@ func (w *watcher) ContainerStarted(id string) {
 			if err := w.weave.RegisterWithDNS(id, fqdn, net.IPAddress); err != nil {
 				w.driver.warn("ContainerStarted", "unable to register %s with weaveDNS: %s", id, err)
 			}
-			if err := common.ConfigureARPforVeths(info.State.Pid, weavenet.VethName); err != nil {
+			if _, err := weavenet.WithNetNSByPid(info.State.Pid, "configure-arp", weavenet.VethName); err != nil {
 				w.driver.warn("ContainerStarted", "unable to configure interfaces: %s", err)
 			}
 		}