Enable IPv6 in docker containers

Python unit tests for some OpenStack components (esp. Neutron) require
enabled IPv6 for test environment (inside a docker container).

Here is an example job build:

  https://ci.mcp.mirantis.net/job/pike-openstack-test-py27-xenial/5/

Change-Id: Idcf4a8281dffb9382cc697a5b5e004c14f5be209
1 file changed