Support for newer iproute
Support for newer iproute

Newer iproute provides id along with netns name in 'ip netns list', so
let's change how we check that our ARP netns exists.

openstack/networking-bagpipe-l2,openstack/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2