Adds a mixed IPv4/IPv6 members traffic test

This patch adds a traffic scenario test for a load balancer with one
IPv4 member and one IPv6 member.
It also makes sure the cirros web servers enable the IPv6 address
assigned to them.

Story: 1627892
Task: 5311
Depends-On: https://review.openstack.org/611460

Change-Id: Ic640c89b5a6ef0d6aade386a910b0d023520aedc
2 files changed