check if extra_alerts is defined in model

Change-Id: I8bfa1088d48273d194194c9ee27d89fde6eebf53
diff --git a/ceph/meta/prometheus.yml b/ceph/meta/prometheus.yml
index 4993289..a8755ff 100644
--- a/ceph/meta/prometheus.yml
+++ b/ceph/meta/prometheus.yml
@@ -100,7 +100,7 @@
       annotations:
         summary: "{{100*threshold}}% of Ceph pool space is used"
         description: "The Ceph {{pool_name}} pool uses {{100*threshold}}% of available space for 3 minutes. For details, run 'ceph df'."
-    {%- if monitoring.cluster_stats.extra_alerts.get("enabled", False) %}
+    {%- if monitoring.cluster_stats.extra_alerts is defined and monitoring.cluster_stats.extra_alerts.get("enabled", False) %}
     CephPool{{pool_name|replace(".", "")|replace("-", "")}}WriteOpsTooHigh:
       {%- set threshold = monitoring_pool.pool_write_ops_threshold|default('200')|float %}
       if: >-