Fix RabbitMQTooMuchUnack alert description

Using `sum(rabbitmq_queue_messages_unacked)` means we loose all labels,
so `{{ $labels.instance }}` in the description was empty. Fix by
checking the number of messages on each host instead and lower the
threshold to 100.

Also, the metric rabbitmq_queue_messages_unacked has no `queue` label,
so `BY (queue)` is useless.

Author: priteau

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

@@ -42,7 +42,7 @@ groups:
     annotations:
       description: RabbitMQ file descriptors usage on {{ $labels.instance }}
   - alert: RabbitMQTooMuchUnack
-    expr: sum(rabbitmq_queue_messages_unacked) BY (queue) > 1000
+    expr: rabbitmq_queue_messages_unacked > 100
     for: 1m
     labels:
       severity: warning