all required vars have values in aks-as-mgmt.sh

[jackfrancis]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

While trying to go through the AKS tilt flow I got an unbound variable error that is fixable by providing a default value.

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

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

NONE

[k8s-ci-robot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.66%. Comparing base (976f893) to head (39b0748).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5197   +/-   ##
=======================================
  Coverage   52.66%   52.66%           
=======================================
  Files         273      273           
  Lines       29189    29189           
=======================================
  Hits        15371    15371           
  Misses      13029    13029           
  Partials      789      789           

☔ View full report in Codecov by Sentry.
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[k8s-ci-robot]

@jackfrancis: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-azure-e2e	39b0748	link	true	/test pull-cluster-api-provider-azure-e2e

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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