test: configurable CONTAINER_RUNTIME via E2E

[jackfrancis]

Reason for Change:

This PR enables ad hoc container runtime configuration injection across the set of our test cluster configuration matrix.

Also, removing the flannel + docker configuration, as it doesn't have a feature sponsor, and it stopped working anyways starting w/ k8s 1.16.

Issue Fixed:

Requirements:

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

Notes:

[acs-bot]

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[codecov]

Codecov Report

Merging #2753 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2753   +/-   ##
=======================================
  Coverage   72.35%   72.35%           
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  Files         137      137           
  Lines       25341    25341           
=======================================
  Hits        18335    18335           
  Misses       5949     5949           
  Partials     1057     1057