contrib/kube: Add rbac role to discover prometheus

The current example-app setup in the kube-prometheus project is able to
discover scraping targets in the default namespace. It is not able to
discover the configured Alertmanager in the monitoring namespace.

This patch adds an alertmanager-discovery rbac role, to permit the
above described action. In addition it does the following cleanups:

- Remove kubeconfig configuration in deploy and teardown script. kubectl
chooses .kube/config whenever KUBECONFIG is not set by default

- Remove namespace specification option via NAMESPACE env var. In most
of the manifests the metadata/namespace was hardcoded anyways, in
addition in the promtheus frontend role binding the service account
namespace is hardcoded to default as well.

- Instead of `kubectl {apply,delete}` individual manifests, the deploy
and teardown shell scripts {apply,delete} on the entire folder.