Add nodeLocalDNSCache.kubeDnsOnly option

[javipolo]

Add a kubeDnsOnly option for nodeLocalDNScache

Add ability to use kube-dns as upstream on node-local-dns, instead of using systems default in resolv.conf

With this flag enabled, even if we use the node-local-dns to cache responses, we use as upstream the main kube-dns server for all types of requests, and not only the ones of *.cluster.local

This way we can customize the main kube-dns for external domains and make this changes also effective to the node-local-dns nodes

Example use case

In our case, we're using rewrites on coredns to easily route some requests to internal services without exiting the cluster, and without modifying the application. With this flag we can keep this behaviour and still use node-local-dns:

rewrite name exact queue.myapp.com queue-system.myapp.svc.cluster.local

A dns request of queue.myapp.com to coredns will return the internal IP address of the service queue-system in myapp namespace

So far when enabling node-local-dns in kops this behaviour dissapeared and queue.myapp.com was resolving to an external internet IP address.
With this patch we can decide if to route all traffic to internal kube-dns, or keep the actual behaviour

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[k8s-ci-robot]

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[javipolo]

@olemarkus I did the proposed changes 👍

[johngmyers]

Go convention is to have acronyms in all-caps.

[johngmyers]

/ok-to-test

[hakman]

@johngmyers Not quite sure the field name is appropriate. Any thoughts?

[johngmyers]

You have a point. It's more like ForwardToKubeDNS.

[javipolo]

I saw "Dns" somewhere in the code so I thought it was OK
Anyways I'm pretty happy to change both the acronym and the name of the field, to be honest finding a proper name is the hardest task haha

Here's the new PR

[johngmyers]

/lgtm
/approve

[johngmyers]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: javipolo, johngmyers

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[jorge07]

Thanks for the work Javi and folks