dhcp: Add priority option to dhcp.

Currently, we can not set the metric of routes in dhcp.
It's ok if there is only a network interface.

But if there are multiple network interfaces, and both have a default route,
We need to set the metric of the route to make the traffic
go through the correct network interface.

For host-local and static, we can set the metric with the route.priority option.
But there is no such option for dhcp.

Signed-off-by: Songmin Li <lisongmin@protonmail.com>

plugins/ipam/dhcp/daemon.go

@@ -109,6 +109,11 @@ func (d *DHCP) Allocate(args *skel.CmdArgs, result *current.Result) error {
 		Gateway: l.Gateway(),
 	}}
 	result.Routes = l.Routes()
+	if conf.IPAM.Priority != 0 {
+		for _, r := range result.Routes {
+			r.Priority = conf.IPAM.Priority
+		}
+	}
 
 	return nil
 }