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.

manifests/examples/example-app/prometheus-frontend-alertmanager-discovery-role-binding.yaml

@@ -0,0 +1,13 @@
+apiVersion: rbac.authorization.k8s.io/v1beta1
+kind: RoleBinding
+metadata:
+  name: prometheus-frontend
+  namespace: monitoring
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: Role
+  name: alertmanager-discovery
+subjects:
+- kind: ServiceAccount
+  name: prometheus-frontend
+  namespace: default