Allow users to configure DNS monitoring
Allow users to configure DNS monitoring

**Problem:**
Doesn't allow user to configure DNS monitoring

**Solution:**
Remove `false` configuration item from mandatory option:
- if interested in kube-dns metrics (v1.12)
    + configuring `exporter-kube-dns.enabled` item as `true` when
    enabling cluster-level monitoring
    + guaranteeing the DNS service of `kube-system` namespace has:
        - `k8s-app=kube-dns` label
        - `dnsmasq-metrics` port with `TCP:10054`
        - `skydns-metrics` port with `TCP:10055`
- if interested in coredns metrics (v1.14)
    + configuring `exporter-coredns.enabled` item as `true` when enabling
    cluster-level monitoring
    + guaranteeing the DNS service of `kube-system` namespace has:
        - `k8s-app=kube-dns` label
        - `metrics` port with `TCP:9153`

**Issue:**
https://github.com/rancher/rancher/issues/21066

rancherio/rancher,cjellick/rancher,rancher/rancher,rancherio/rancher,rancher/rancher,cjellick/rancher,rancher/rancher,rancher/rancher,cjellick/rancher