Bridges should not have any IPv6 addresses, either
link-local, or on the tenant's private network due
to the bridge processing Router Advertisements from
Neutron and auto-configuring addresses, since it will
allow access to the hypervisor from a tenant VM.
The bridge only exists for the Neutron security group
code, so we can safely disable IPv6 on it.
Similar to other nova and neutron changes such as
https://review.openstack.org/#/c/313070/
Change-Id: I2ce540b4201cd227c5df53daeb686c4cc49a6e59
Closes-bug: 1302080
c74f92a0e0