title: Alertmanager only goes though default receiver
body: <p>As stated in the title, I have an issue with alertmanager from prometheus-operator. I'm trying to filter some alerts using routes but it doesn't seem to be taken into account.</p>
  <p>When I use the default receiver everything seems to be working fine but if I try to filter with a default receiver that does nothing and then a route, it doesn't goes in the route and I don't get any error.</p>
  <p>The secret I'm using:</p>
  <pre><code>route:
    group_wait: 30s
    group_interval: 5m
    repeat_interval: 4h
    group_by: [cluster]
    receiver: 'default'
    routes:
    - match:
        alertname: !Watchdog
      receiver: 'slack-devops'
      continue: true
  templates: ['/etc/alertmanager/config/*.tmpl']
  receivers:
  - name: 'default'
  - name: 'slack-devops'
    slack_configs:
    - channel: &quot;tmp-test-srv-alerting&quot;
  </code></pre>
  <p>With this configuration it puts everything in the default route and netheir goes in the routes, I don't even get an error. Would someone has an idea on how to make this work?</p>