Only deliver severity=page alerts

Alerts of other severity levels are kept as advisory and not routed
anywhere.

roles/prometheus/templates/alertmanager.yml.j2

@@ -16,6 +16,10 @@ route:
   repeat_interval: 12h
   receiver: default
 
+  # Only severity=page alerts are ever sent anywhere.
+  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
 # the blackbox prober metrics).