The `prometheus-frontend` role of the example app kubeprometheus section
is scoped to the default namespace. Thereby the frontend Prometheus
instance is not able to discover anything outside of the default
namespace. We might as well restrict the front end service monitor to
the default namespace too.

added service account, role and role binding for the prometheus frontend
example, also updated prometheus to use the correct service account

fixes #1049

This updates prometheus to 1.7.1 where it was missing

* namespace has to be kube-system

In order for this service to be discovered by ServiceMonitor k8s-apps-http, it should belong to namespace kube-system.

* namespace has to be kube-system

* namespace has to be kube-system

for service discovery to work within prometheus configuration (ServiceMonitor k8s-app) these service and endpoints should belong to kube-system namespace. Otherwise the service monitor won't fetch it.

* selector was incorrect

selector prometheus: prometheus-frontend is incorrect, the name of the Prometheus resource in the example is "frontend", not "prometheus-frontend"

selector prometheus: prometheus-frontend is incorrect, the name of the Prometheus resource in the example is "frontend", not "prometheus-frontend"

git-subtree-dir: contrib/kube-prometheus
git-subtree-mainline: 050ca21276696c8603375c699513ec487301ed62
git-subtree-split: 81c0d2f4