AAP stops working when VM is restarted

AAP stops working when VM is restarted

Issue:
========
When VM's are having a static-ip without VRRP
configuration. When such VMs are rebooted, L3VPN will get interface-state
as DOWN. L3VPN service removed the adjacency from config-vpn-interface.
This triggers update of VpnInterface listener and eventually FIB is
removed for the static-ip. When VM comes-up , if static-ip is pinged ,
L3VPN service tries to learn the static-ip again by sending ARP Broadcast.
When it receives the ARP Response, it sees that NeutronVpnPortIpToPort
entry is present and mac-addr/interface is same. Hence no action taken.
So discovery of this static-ip fails and FIB is already removed before.
So eventually traffic for the static-ip fails either way.

Solution:
==========
When VM is rebooted, upon receiving interface-state DOWN, L3VPN will not
remove static-ip adjacency and FIB. Only when interface-state remove is
received, L3VPN will remove the static-ip adj, FIB, NeutronVpnPortIp etc.

Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>
Change-Id: I8198f9bc8099818ec9a3f45836246fe2b89a5778