fix: scale cannot find VM index for the VMAS+VHD+Linux case

[jadarsie]

Reason for Change:
Scaling fails if the target pool is a VMAS pool of VHD-based linux nodes.
VM osPublisher is not enought to distinguish between Linux and Windows.

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

Notes:

[marosset]

/lgtm
@mboersma or @jackfrancis can you take a look too? - this logic has been fragile

[codecov]

Codecov Report

Merging #2906 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #2906      +/-   ##
=========================================
+ Coverage    72.5%   72.5%   +<.01%     
=========================================
  Files         141     141              
  Lines       25744   25743       -1     
=========================================
  Hits        18666   18666              
+ Misses       5998    5997       -1     
  Partials     1080    1080

[jackfrancis]

@jadarsie Let's ensure we protect any fixes to VMAS + scale by adding "SCALE_CLUSTER": true to this test config:

https://github.com/Azure/aks-engine/blob/master/test/e2e/test_cluster_configs/availabilityset.json#L3

(under the env config object)

[marosset]

Lets add a windows node pool to the ^^^ config

[jackfrancis]

/lgtm

[acs-bot]

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This pull-request has been approved by: jackfrancis, jadarsie, marosset

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