small changes in k8s/minikube and example-app (#392)

* 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"