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

inventory/sample/group_vars/k8s-cluster.yml

@@ -163,9 +163,15 @@ dashboard_enabled: true
 # Monitoring apps for k8s
 efk_enabled: false
 
-# Helm deployment
+# Helm deployment. Needs for Prometheus Operator, k8s metrics.
 helm_enabled: false
 
+# Prometheus Operator. Needs for k8s metrics. Installed Helm is required.
+prometheus_operator_enabled: false
+
+# K8s cluster metrics. Installed Helm and Prometheus Operator are required.
+k8s_metrics_enabled: false
+
 # Istio deployment
 istio_enabled: false