manifests: add Alertmanager manifests and discovery

The latest version of the Prometheus Operator requires Prometheus
>=v1.4.0 for the Alertmanger discovery feature.

manifests/alertmanager/alertmanager-config.yaml

@@ -0,0 +1,18 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: alertmanager-main
+data:
+  alertmanager.yaml: |-
+    global:
+      resolve_timeout: 5m
+    route:
+      group_by: ['job']
+      group_wait: 30s
+      group_interval: 5m
+      repeat_interval: 12h
+      receiver: 'webhook'
+    receivers:
+    - name: 'webhook'
+      webhook_configs:
+      - url: 'http://alertmanagerwh:30500/'