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