Prevent KubeletDown alerts on custom project monitoring setup

[groundhog2k]

Default monitoring setup for CaaS projects shows "KuebeletDown" alerts in alertmanager. @eumel8 we spoke abot that in chat.
To prevent that, the KubeletDown alert should be disabled by default, which is prepared with this change.

[puffitos]

LGTM. Interestingly enough, disabling the kubelet doesn't seem to remove all kubelet related alerts (I didn't do a deep-dive to search where they are created):

KubeletTooManyPods (0 active)
KubeletPlegDurationHigh (0 active)
KubeletPodStartUpLatencyHigh (0 active)
KubeletClientCertificateExpiration (0 active)
KubeletClientCertificateExpiration (0 active)
KubeletServerCertificateExpiration (0 active)
KubeletServerCertificateExpiration (0 active)
KubeletClientCertificateRenewalErrors (0 active)
KubeletServerCertificateRenewalErrors (0 active)

[groundhog2k]

@puffitos I think this is ok. Because we only see "red alert" for KubeletDown, all the other metrics are "green". I didn't want to kill them all if not necessary. -- Info: All the kubelet alerts are defined in https://github.com/prometheus-community/helm-charts/blob/main/charts/kube-prometheus-stack/templates/prometheus/rules-1.14/kubernetes-system-kubelet.yaml