lib/neutron-legacy: replace ip when re-stacking
lib/neutron-legacy: replace ip when re-stacking

Replicated Yi Zhao's fix for re-adding ipv6 addresses to neutron-legacy
(review I9ff62023dbc29a88aec3c48af331c0a49a1270bb).
Previously, re-stacking failed with "File exists" for ipv6 addresses
on br-ex.  With this change, the existing address is replaced on
br-ex with the appropriate address.

Change-Id: I6e6235132a34469f4e68b5bb3cf51ebdf01c83a2

openstack-dev/devstack,dtroyer/devstack,group-policy/devstack,bswartz/devstack,group-policy/devstack,nawawi/openstack,nawawi/openstack,bswartz/devstack,richliu/devstack,openstack-dev/devstack,noironetworks/devstack,dtroyer/devstack,richliu/devstack,noironetworks/devstack