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
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.
