ipvlan: Send Gratuitous ARP after IPs are set

[TothFerenc]

This PR fixes the missing Gratuitous ARP functionality in the IPVLAN plugin.
The code is copied from the MACVLAN plugin:

plugins/plugins/main/macvlan/macvlan.go

Lines 297 to 311 in e27c48b

	if err := ipam.ConfigureIface(args.IfName, result); err != nil {
	return err
	}
	contVeth, err := net.InterfaceByName(args.IfName)
	if err != nil {
	return fmt.Errorf("failed to look up %q: %v", args.IfName, err)
	}
	for _, ipc := range result.IPs {
	if ipc.Address.IP.To4() != nil {
	_ = arping.GratuitousArpOverIface(ipc.Address.IP, *contVeth)
	}
	}
	return nil

Fixes #673

[mars1024]

/lgtm, thanks!

[dcbw]

/lgtm

[mccv1r0]

/lgtm

[TothFerenc]

@dcbw Thanks for the merge!
Do you plan a release anytime soon?