feat: add warning message for empty Location string

[jackfrancis]

Reason for Change:

This PR introduces a stdout warning message when AKS Engine template generation observes an empty "location" value in the input api model. We want to tell folks pro-actively that AKS Engine will treat that empty string significantly, and that the resultant ARM templates will work exclusively for public cloud regions.

Issue Fixed:

Addresses part of #3170

Requirements:

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

Notes:

[jackfrancis]

@gball14 FYI

[jackfrancis]

@gurudatttest FYI

[codecov]

Codecov Report

Merging #3174 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3174   +/-   ##
=======================================
  Coverage   71.23%   71.23%           
=======================================
  Files         147      147           
  Lines       25745    25747    +2     
=======================================
+ Hits        18340    18342    +2     
  Misses       6268     6268           
  Partials     1137     1137           

Impacted Files	Coverage Δ
pkg/api/vlabs/validate.go	80.19% <100.00%> (+0.03%)	⬆️

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[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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