SYMC: deleting subnet port from router removes ZK reservation

On neutron add subnet to router,
In add_router_interface
https://github.com/Juniper/contrail-controller/blob/master/src/config/vnc_openstack/vnc_openstack/neutron_plugin_db.py#L3198

A port-create with a fixed ip is simulated. This ends up creating an instance-ip for port with gw-ip-address. Ideally this shouldn't create an instance-ip.

Similarly i remove_router_interface, port-delete is issued and this releases the above instance-ip causing the zk node to be deleted for gw-ip-address.

This doesn't cause any harm in ip-alloc-req because vnc_addr_mgmt.py uses Subnet() which doesn't allow to pick from gw-ip. But if user had specified gw-ip in port-create then it will be treated as available and used.