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