Fix alertmanager config

The root route must not have matchers, create a child one instead.

roles/prometheus/templates/alertmanager.yml.j2

@@ -14,11 +14,15 @@ route:
   group_wait: 30s
   group_interval: 5m
   repeat_interval: 12h
-  receiver: default
+
+  # No delivery of alerts by default.
+  receiver: null
 
   # Only severity=page alerts are ever sent anywhere.
-  match:
-    severity: page
+  routes:
+    - receiver: 'alert-email'
+      match:
+        severity: page
 
 # The following inhibit rules are meant to work both with the job/host
 # hierarchy (for standard metrics) and with the probe/target one (for
@@ -44,7 +48,8 @@ inhibit_rules:
     equal: ['host']
 
 receivers:
-  - name: default
+  - name: null
+  - name: 'alert-email'
     email_configs:
       - to: '{{ alert_email }}'