fix: add toleration to run flannel on not-yet-ready nodes.

[josecv]

Without this toleration, a v1.12 aks-engine cluster using flannel as its
network plugin will be unable to come up, as the cni plugin will never
start running on the nodes.

Reason for Change:

Currently when an v1.12 aks-engine cluster comes up, the nodes start out with a not-ready taint. The flannel daemonset does not tolerate this taint, so flannel can not come up. This in turn prevents the nodes from ever going ready.

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:
I'm not super sure how we could document or unit test this, but feel free to let me know how we could add that to this PR.

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[josecv]

/assign @CecileRobertMichon

[codecov]

Codecov Report

Merging #313 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #313   +/-   ##
=======================================
  Coverage   53.16%   53.16%           
=======================================
  Files          95       95           
  Lines       14244    14244           
=======================================
  Hits         7573     7573           
  Misses       6006     6006           
  Partials      665      665

[jackfrancis]

Thanks @josecv! FYI I rebased this to bring it up to master HEAD, in order to perform some flannel-specific tests in the background. Change lgtm, will merge pending test results.

[jackfrancis]

/lgtm

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