Comment 13 for bug 1649586

Finally found the root cause. This patch forces grenade to set cells v2

https://github.com/openstack-dev/grenade/commit/ecfaa7a5a235a69a0ead8b53dd4b2807093c9cfa

After this commit LM+grenade job started to fail every time. This is because grenade forces RABBIT_HOST to be localhost on controller node

https://github.com/openstack-dev/grenade/blob/b7ae1b46602e34d53b5489ba9e0a7cb73d6ea418/devstack.localrc.base#L21
https://github.com/openstack-dev/grenade/blob/b7ae1b46602e34d53b5489ba9e0a7cb73d6ea418/devstack.localrc.target#L21

So when nova configures cells v2 deployment it uses localhost in transport_url and saves such url in cell mapping. When subnode-2 wants to start pre live migration steps on newer node, it takes cell_mapping from database and uses transport url which points to localhost. Unfortunately rabbit is not running on the same node, therefore every time LM+grenade job fails on starting pre live migration steps.