prometheus operator, metrics for k8s cluster

install using Helm:
- Prometheus Operator
- metrics for k8s cluster including: grafana dashboard, alertmanager, node exporters

base project:
https://github.com/coreos/prometheus-operator

the issue:
https://github.com/kubernetes-incubator/kubespray/issues/2042

Previous PR, raw ansible without Helm:
https://github.com/kubernetes-incubator/kubespray/pull/2499

roles/kubernetes-apps/meta/main.yml

@@ -27,6 +27,12 @@ dependencies:
       - apps
       - registry
 
+  - role: kubernetes-apps/metrics
+    when: prometheus_operator_enabled
+    tags:
+      - apps
+      - metrics
+
   # istio role should be last because it takes a long time to initialize and
   # will cause timeouts trying to start other addons.
   - role: kubernetes-apps/istio