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
diff --git a/docker/host.yml b/docker/host.yml
index 7e5fb8c..c3cc808 100644
--- a/docker/host.yml
+++ b/docker/host.yml
@@ -10,3 +10,6 @@
       insecure_registries:
         - ${_param:cluster_vip_address}:5000
         - ${_param:cluster_public_host}:5000
+      options:
+        ipv6: true
+        fixed-cidr-v6: fc00::/7