Add a Pod Disruption Budget on kube-dns

[davidmccormick]

Improve the handling of kube-dns during node drains / cluster rolls by defining a pod disruption budget that allows 1 instance to be down.

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[codecov-io]

Codecov Report

Merging #1467 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1467   +/-   ##
=======================================
  Coverage   37.88%   37.88%           
=======================================
  Files          75       75           
  Lines        4593     4593           
=======================================
  Hits         1740     1740           
  Misses       2611     2611           
  Partials      242      242

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[mumoshu]

Awesome! LGTM. Thanks for your contribution as always ☺️