E2E: clusterFirst DNS liveness probe

[jackfrancis]

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

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• documentation
• unit tests
• tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

E2E: clusterFirst DNS liveness probe

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[codecov]

Codecov Report

Merging #4002 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4002   +/-   ##
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  Coverage    50.4%   50.4%           
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  Files         109     109           
  Lines       16856   16856           
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  Hits         8497    8497           
  Misses       7574    7574           
  Partials      785     785

[tariq1890]

LGTM. Are we going to merge this?

[jackfrancis]

I think it's unfinished. The problem reported was that ClusterFirst DNS + external resolvers was broken. So we want to add the external resolvers.

[CecileRobertMichon]

@jackfrancis did this make it to https://github.com/Azure/aks-engine?