Adding another way to flush ARP cache

This is a workaround for the case I've described [here](http://serverfault.com/questions/599799/ip-address-reuse-on-macvlan-devices). I've added _ping_ to force gateway to update ARP cache entry for IP of macvlan interface. It is done on first packet with new MAC:IP addresses, looks like just _arping_ is not enough.