store Azure CNI version in api model

[jackfrancis]

What this PR does / why we need it: Stores the statically assigned Azure CNI version in the api model so that it is more easily determined which version of Azure CNI a cluster was built with.

Note: this does not enable configurable versioning of Azure CNI at deployment time.

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

Special notes for your reviewer:

If applicable:

• documentation
• unit tests
• tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

store Azure CNI version in api model

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[jackfrancis]

@weinong @sharmasushant FYI

[CecileRobertMichon]

What about making it a variable in the ARM template? Only reason I'm saying that is it might be confusing for users to have a field in the apimodel / in their cluster configuration that gets ignored. From what I understand we're not allowing this value to be configurable for now.

[jackfrancis]

@CecileRobertMichon I'm open to rationalizing this whole thing (including making the version configurable) and reflecting that in the ARM template. For now this incremental change is useful.

[codecov]

Codecov Report

Merging #3804 into master will increase coverage by 0.05%.
The diff coverage is 85.71%.

@@            Coverage Diff            @@
##           master   #3804      +/-   ##
=========================================
+ Coverage   55.65%   55.7%   +0.05%     
=========================================
  Files         109     109              
  Lines       16233   16240       +7     
=========================================
+ Hits         9034    9047      +13     
+ Misses       6410    6402       -8     
- Partials      789     791       +2