addons: Add prometheus alertmanager flag

* Pass -alertmanager.url to work with a user's in-cluster
alertmanager deployment, if any

addons/prometheus/deployment.yaml

@@ -21,6 +21,7 @@ spec:
           - '-config.file=/etc/prometheus/prometheus.yaml'
           - '-storage.local.retention=12h'
           - '-storage.local.memory-chunks=500000'
+          - '-alertmanager.url=http://alertmanager:9093'
         ports:
         - name: web
           containerPort: 9090