Consider adding additional configuration options for DNS providers

[dkeightley]

Opening this to consider adding information or link for additional DNS provider configuration.

For example, to persist custom CoreDNS ConfigMaps throughout cluster upgrades (by default resets the ConfigMap) it is possible to do this in-line

Example:

rancher_kubernetes_engine_config:
[...]
  addons: |-
      ---
      apiVersion: v1
      kind: ConfigMap
      metadata:
        name: coredns
        namespace: kube-system
      data:
        Corefile: |
          .:53 {
              errors
              log
              health {
                lameduck 5s
              }
              ready
              kubernetes cluster.local in-addr.arpa ip6.arpa {
                pods insecure
                fallthrough in-addr.arpa ip6.arpa
              }
              prometheus :9153
              forward . "/etc/resolv.conf"
              cache 30
              loop
              reload
              loadbalance
          }

[SheilaghM]

@dkeightley - does this apply to v2.4.x only or for both 2.4 and 2.5?