LBaaS VIP doesn't work after delete and re-add

When delete and re-add the same vip, we need to send gratuitous ARP
to flush the ARP cache in the Router.

Change-Id: Id97088abb95f4433a100abdae8c8726b3be42ed2
Closes-Bug: 1301165
Merge "LBaaS VIP doesn't work after delete and re-add"