test: disable useManagedIdentity default value on Azure Stack

[haofan-ms]

Reason for Change:

The default true value of useManagedIdentity adds additional unsupported KubernetesConfig to Azure Stack API model, this is causing failure of Azure Stack E2E runs.

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #4063 (bfcd9e0) into master (254f95f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4063   +/-   ##
=======================================
  Coverage   73.19%   73.19%           
=======================================
  Files         135      135           
  Lines       20555    20555           
=======================================
  Hits        15046    15046           
  Misses       4539     4539           
  Partials      970      970           

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[jackfrancis]

/lgtm