Bug 1760941

This RFE tracks support for the following pattern which is relevant to running multiple instances of Glance accross multiple edge sites:

1. deploy central site with internal ceph1 and glance1 with rbd backend ceph1
2. deploy edge site with internal ceph2 and glance2 with rbd backend ceph2
3. deploy edge site with internal ceph3 and glance3 with rbd backend ceph3
4. deploy edge site with internal cephN and glanceN with rbd backend cephN
5. update central site with internal ceph1, external ceph2, ceph3, ..., cephN and glance1 with rbd backends ceph1, ceph2, ceph3, ..., cephN

Steps 1-4 are already possible through split control plane deployments [0], but step 5 is not possible because a single stack only deals with a single ceph cluster. This RFE is focussed only on step 5's support for multiple external ceph clusters in a single stack update (internal ceph1, external ceph2, ceph3, ..., cephN). Though glance supports multiple backends [1], a separate RFE could track enabling TripleO to configure them (glance1 with rbd backends ceph1, ceph2, ceph3, ..., cephN). More details on configuring multiple external Ceph backends is filed in an upstream Tripleo Blueprint [2].

During step 5 we should support (and document) which ansible tags to pass when using config-download in order to minimize change so that the central stack update minimizes service restarts.

[0] https://docs.openstack.org/project-deploy-guide/tripleo-docs/latest/features/distributed_compute_node.html

[1] https://docs.openstack.org/glance/rocky/admin/multistores.html

[2] https://blueprints.launchpad.net/tripleo/+spec/multiple-external-ceph