Make cmd more testable by abstracting authArgs and AzureClient

[serbrech]

What this PR does / why we need it:
Improve testability of the cmd package by abstracting the authArgs dependency.
This allows tests to stub the authArgs.getClient() call and return a fake ACSEngineClient instead of the concrete AzureClient instance

Special notes for your reviewer:
If this is ok, the same pattern can be applied for other command that depends on the AzureClient.

[codecov]

Codecov Report

Merging #3756 into master will increase coverage by 0.31%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #3756      +/-   ##
==========================================
+ Coverage   55.48%   55.79%   +0.31%     
==========================================
  Files         108      108              
  Lines       16140    16143       +3     
==========================================
+ Hits         8955     9007      +52     
+ Misses       6420     6347      -73     
- Partials      765      789      +24

[jackfrancis]

/lgtm pending E2E, thanks @serbrech!

[jackfrancis]

@serbrech FYI, this looks fine, but it occured to me that our E2E doesn't traverse bin/acs-engine deploy and instead uses bin/acs-engine generate + az group deployment create.

Let's use this as an excuse to build a 1st class bin/acs-engine deploy E2E test. Stand by... :)

[serbrech]

added #3776

[CecileRobertMichon]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon, jackfrancis, serbrech

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