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/example-app.yaml

@@ -4,6 +4,7 @@ metadata:
   name: example-app
   labels:
     tier: frontend
+  namespace: default
 spec: 
   selector: 
     app: example-app 
@@ -17,6 +18,7 @@ apiVersion: extensions/v1beta1
 kind: Deployment
 metadata:
   name: example-app
+  namespace: default
 spec:
   replicas: 4
   template: