chore: change MTU only if not Azure CNI on Azure Stack

[jadarsie]

Reason for Change:

Azure CNI is not using encap, no need to reduce MTU on Azure Stack

Issue Fixed:

Related #1346

Requirements:

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

[jackfrancis]

nit: unnecessary indentation

(the pattern we've been following thus far is not to indent actual static chars [e.g., the shell script in this case], so you can put the {{ and the echo statements on a common indentation indentation alignment at the beginning of the line)

[jackfrancis]

/lgtm

[jackfrancis]

@jadarsie let me know when you've tested this successfully in an Azure Stack environment, and I'll merge

[codecov]

Codecov Report

Merging #3367 into master will decrease coverage by 1.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3367      +/-   ##
==========================================
- Coverage   72.16%   70.96%   -1.20%     
==========================================
  Files         147      147              
  Lines       25419    26873    +1454     
==========================================
+ Hits        18343    19070     +727     
- Misses       5939     6563     +624     
- Partials     1137     1240     +103     

Impacted Files	Coverage Δ
pkg/engine/templates_generated.go	47.14% <ø> (+2.66%)	⬆️
pkg/api/vlabs/validate.go	80.37% <0.00%> (-0.16%)	⬇️
pkg/api/defaults.go	92.58% <0.00%> (-0.07%)	⬇️

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[jadarsie]

@jadarsie let me know when you've tested this successfully in an Azure Stack environment, and I'll merge

this is good to go

[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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