rbd: start using active-active

This patch drops the hacky workaround of using backend_host which
is not recommended by the Cinder team and instead uses active-active
RBD which has been implemented since Rocky.

Closes-Bug: #1837403
Change-Id: I0c8aed4d0608c1f117e1baa1f428875956159ffd

releasenotes/notes/enable-active-active-9af1551759468dc8.yaml

@@ -0,0 +1,12 @@
+---
+features:
+  - Cinder is deployed with Active-Active enabled by default if you are using
+    Ceph as a backend storage.
+issues:
+  - The previous way of using a common backend_host across all deployments
+    was not recommended by the Cinder team and it will cause duplicate
+    messages that cause problems in the environment.
+upgrade:
+  - It is possible that you may need to use the cinder-manage command to migrate
+    volumes to a specific host.  In addition, you will have to remove the old
+    ``rbd:volumes`` service which will be stale.

templates/cinder.conf.j2

@@ -14,6 +14,10 @@ rootwrap_config = /etc/cinder/rootwrap.conf
 api_paste_config = /etc/cinder/api-paste.ini
 auth_strategy = {{ cinder_auth_strategy }}
 
+{% if cinder_backend_rbd_inuse | bool %}
+cluster = ceph
+{% endif %}
+
 ## Cinder Scheduler
 {% if cinder_scheduler_default_filters is defined %}
 scheduler_default_filters={{ cinder_scheduler_default_filters | join(',') }}
@@ -101,9 +105,6 @@ allowed_direct_url_schemes = cinder
 {% for backend_section in cinder_backends|dictsort %}
 [{{ backend_section.0 }}]
 {% for key, value in (backend_section.1 | dictsort) if key not in ['extra_volume_types', 'shares'] %}
-{% if key == "volume_driver" and value == "cinder.volume.drivers.rbd.RBDDriver" %}
-backend_host="rbd:volumes"
-{% endif %}
 {{ key }}={{ value }}
 {% endfor %}