fix: addons "update config" is upgrade-only

[jackfrancis]

Reason for Change:

We are unnecessarily re-configuring the api model addons spec on both scale and upgrade operations. We should only be doing this for upgrade operations, as scale operations do not affect the master vms, and thus do not change the control plane implementation.

Issue Fixed:

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #2282 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2282   +/-   ##
=======================================
  Coverage   71.27%   71.27%           
=======================================
  Files         142      142           
  Lines       24471    24471           
=======================================
  Hits        17441    17441           
  Misses       5916     5916           
  Partials     1114     1114

[CecileRobertMichon]

do we also need to update the unit tests https://github.com/Azure/acs-engine/pull/4224/files#diff-daf624fb49e7cf74e670db52d41e949a ?

can you run a backcompat scale test on this (with a release before PR 4224 merged) just to make sure we're not missing anything?

[jackfrancis]

@CecileRobertMichon in fact the proof that this is the right thing is in not changing the UT and validating everything is still green.

[CecileRobertMichon]

lgtm

[acs-bot]

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