Add proper indentation to extra CoreDNS config

[javipolo]

When using more than one line of extra config, the indentation was not right and generated YAML was not valid

This should fix this issue also for 0.12.x (we're currently migrating away from it, but it feels right to leave a working version in the repo)

[k8s-ci-robot]

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[codecov-io]

Codecov Report

Merging #1814 into v0.12.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           v0.12.x    #1814   +/-   ##
========================================
  Coverage    38.19%   38.19%           
========================================
  Files           75       75           
  Lines         4658     4658           
========================================
  Hits          1779     1779           
  Misses        2633     2633           
  Partials       246      246

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