Split Control Plane Unified Ansible Inventory

By design split-control-plane deployments [1] isolate stacks but sometimes it is useful to have ansible plays which run processes across stacks. For example, given the following deployment:

- central control plane
- edge site1
- edge site 2
- edge site 3

Where glance cache [2] is deployed to each edge site, there is a usecase to precache the glance images from the central site to edge sites 1 and 2, but not 3. The precaching operation for image foo can be achieved by running glance and/or nova commands on edge nodes. Running these commands across all edge nodes via Ansible will be useful for operators.

We should provide a means to generate an ansible inventory which includes all nodes from the dpeloyment above, except form site 3, from TripleO where the config-download tools query each stack to create the inventory.

Playbooks which fulfill this function could then be provided in the tripleo-ansible repository [3].

[1] https://blueprints.launchpad.net/tripleo/+spec/split-controlplane
[2] https://blueprints.launchpad.net/tripleo/+spec/split-controlplane-glance-cache
[3] https://github.com/openstack/tripleo-ansible