Allow extra Coredns configuration

[jorge07]

When touching the coredns configmap a cluster upgrade will overwrite the changes. This PR introduces a way to configure the changes inside the cluster config so no need to touch manually the config.

It will be ignored when other provider is specified

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[jorge07]

Is this is accepted I'll need to port this to +0.1X versions

[codecov-io]

Codecov Report

Merging #1695 into v0.12.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           v0.12.x    #1695      +/-   ##
===========================================
+ Coverage    38.17%   38.19%   +0.01%     
===========================================
  Files           75       75              
  Lines         4657     4658       +1     
===========================================
+ Hits          1778     1779       +1     
  Misses        2633     2633              
  Partials       246      246

Impacted Files	Coverage Δ
core/controlplane/config/config.go	63.28% <100%> (+0.04%)	⬆️

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[davidmccormick]

Thanks very much for this contribution - could you also PR this for master, 13.x and 14.x branches?

[jorge07]

Sure, will do