[v0.13.x] Allow CoreDNS resources to be configured

[dominicgunn]

Changes

• Allows CoreDNS resources to be configured through cluster.yaml.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov-io]

Codecov Report

Merging #1728 into v0.13.x will decrease coverage by 0.04%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           v0.13.x    #1728      +/-   ##
===========================================
- Coverage    25.32%   25.28%   -0.05%     
===========================================
  Files           98       98              
  Lines         5077     5087      +10     
===========================================
  Hits          1286     1286              
- Misses        3649     3659      +10     
  Partials       142      142

Impacted Files	Coverage Δ
pkg/api/types.go	0% <ø> (ø)	⬆️
pkg/api/cluster.go	0% <0%> (ø)	⬆️

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[davidmccormick]

/lgtm