[Feature] CoreDNS: External CoreFile option

[gjtempleton]

Resolves #7347 as discussed at yesterday's office hours. Think this is the approach that was agreed on.

Allow users to specify entirely custom CoreFile contents for CoreDNS.

Provided example and warning(s) for users in the cluster spec.

FYI: I'm likely out of contact until next Saturday (the 12th) or so however I've given @maruina collaborator rights on my fork so he may be able to resolve any queries etc.

[k8s-ci-robot]

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[gjtempleton]

/assign @KashifSaadat

[joshbranham]

I think something like this would be really useful. We are currently just injected the updated ConfigMap after a cluster is created, but it would be great if this was more inline. The only reason I think it should be is because CoreDNS is an add-on (right?) and therefore kops enables it.

[justinsb]

Sorry for delay!

/ok-to-test

[mikesplain]

I think this is great. My team was thinking about something similar.

I was going to ask about whether we may want to consider making the whole thing a template, which you would be overriding, but that would move the complexity into code rather than the template it's self.

Given the value this would add for many to not have to work around kops management of coredns, I'm going to approve this. Thanks so much for the hard work!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gjtempleton, mikesplain

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[mikesplain]

I fixed a conflict since I believe @gjtempleton isn't around this week.

[joshbranham]

Big fan of this as stated above!

/lgtm

[gjtempleton]

Awesome, great to come back from holiday to find this merged and cherry-picked back!

Thanks for the work in fixing the conflicts @mikesplain.