In some cases we have been using `alert-rules` in some cases
`prometheus-rulefiles`. This led to confusion [1]. Instead, unify the
Prometheus rules configmap labels to:

```yaml
labels:
  role: alert-rules
```

[1] https://github.com/coreos/prometheus-operator/issues/1102

This updates prometheus to 1.7.1 where it was missing

In cleanup commit 9cd05a8c39c6ed03688859de92372d93af5cc6e2 the labels on the ServiceMonitors were changed from k8s-apps to k8s-app. This commit makes the Prometheus selector match that change.

For `--dry-run` to work with kubectl a Kubernetes cluster's apiserver is
actually used, which is unnecessary for generating these manifests. This
approach also allows further customization, such as adding labels to the
generated manifests.

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