Create an IPv6 network that uses an external router for IPv6

Change-Id: Id1498d74db816b3edbb8376ca5acb7cc5792ea5c
Merge "Create an IPv6 network that uses an external router for IPv6"
Merge commit 'refs/changes/83/86083/2' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch