fix: use controller length for RabbitMQNodeDown

The `RabbitMQNodeDown` made the assumption that all deployments involve
three controllers. However, this is not always the case as we do support
deployments with a single controller or more than three controllers.

Before this would have caused false alerts in deployments with a single
controller. Whilst also concealing alerts in deployments with more than
three controllers.

Author: jackhodgkiss

etc/kayobe/kolla/config/prometheus/rabbitmq.rules

@@ -6,7 +6,7 @@ groups:
 - name: rabbitmq.rules
   rules:
   - alert: RabbitMQNodeDown
-    expr: sum(rabbitmq_build_info{instance!=""}) < 3
+    expr: sum(rabbitmq_build_info{instance!=""}) < {% endraw %}{{ groups['controllers'] | length }}{% raw %}
     for: 30m
     labels:
       severity: critical

releasenotes/notes/use-length-for-rabbitmq-node-down-rule-c9e9c6b09f57954d.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Use the length of the ``controller`` group to determine if any RabbitMQ
+    nodes are down. This is benefical for deployments that do not use a
+    standard three node setup.