Fix initial prometheus alerts

On a fresh prometheus installation in CI, there are already 6 initial prometheus alerts (see #1155):

• KubeCPUOvercommit #368 (closed)
• KubeMemOvercommiti #368 (closed)
• CoreDNSDown #337 (closed)
• KubeControllerManagerDown #341 (closed)
• KubeSchedulerDown #341 (closed)
• TargetDown #337 (closed)
• KubeJobFailed
• KubeJobCompletion

This is the alerts result:

[
  {
    "labels": {
      "alertname": "KubeCPUOvercommit",
      "severity": "warning"
    },
    "annotations": {
      "message": "Cluster has overcommitted CPU resource requests for Pods and cannot tolerate node failure.",
      "runbook_url": "https://github.com/kubernetes-monitoring/kubernetes-mixin/tree/master/runbook.md#alert-name-kubecpuovercommit"
    },
    "state": "pending",
    "activeAt": "2019-08-29T15:12:18.628829203Z",
    "value": "1.8e-01"
  },
  {
    "labels": {
      "alertname": "KubeMemOvercommit",
      "severity": "warning"
    },
    "annotations": {
      "message": "Cluster has overcommitted memory resource requests for Pods and cannot tolerate node failure.",
      "runbook_url": "https://github.com/kubernetes-monitoring/kubernetes-mixin/tree/master/runbook.md#alert-name-kubememovercommit"
    },
    "state": "pending",
    "activeAt": "2019-08-29T15:12:18.628829203Z",
    "value": "5.248281014760301e-02"
  },
  {
    "labels": {
      "alertname": "CoreDNSDown",
      "severity": "critical"
    },
    "annotations": {
      "message": "CoreDNS has disappeared from Prometheus target discovery.",
      "runbook_url": "https://github.com/kubernetes-monitoring/kubernetes-mixin/tree/master/runbook.md#alert-name-corednsdown"
    },
    "state": "pending",
    "activeAt": "2019-08-29T15:11:34.084974013Z",
    "value": "1e+00"
  },
  {
    "labels": {
      "alertname": "KubeControllerManagerDown",
      "severity": "critical"
    },
    "annotations": {
      "message": "KubeControllerManager has disappeared from Prometheus target discovery.",
      "runbook_url": "https://github.com/kubernetes-monitoring/kubernetes-mixin/tree/master/runbook.md#alert-name-kubecontrollermanagerdown"
    },
    "state": "pending",
    "activeAt": "2019-08-29T15:11:34.084974013Z",
    "value": "1e+00"
  },
  {
    "labels": {
      "alertname": "KubeSchedulerDown",
      "severity": "critical"
    },
    "annotations": {
      "message": "KubeScheduler has disappeared from Prometheus target discovery.",
      "runbook_url": "https://github.com/kubernetes-monitoring/kubernetes-mixin/tree/master/runbook.md#alert-name-kubeschedulerdown"
    },
    "state": "pending",
    "activeAt": "2019-08-29T15:11:34.084974013Z",
    "value": "1e+00"
  },
  {
    "labels": {
      "alertname": "TargetDown",
      "job": "kube-dns",
      "severity": "warning"
    },
    "annotations": {
      "message": "100% of the kube-dns targets are down."
    },
    "state": "pending",
    "activeAt": "2019-08-29T15:11:28.421006645Z",
    "value": "1e+02"
  }
]