Autofill ACI Connector credentials and enable logs

[sozercan]

What this PR does / why we need it:

• autofills required credentials for virtual-kubelet/aci-connector addon (clientId, clientSecret, resourceGroup, tenantId, subscriptionId)
• generates certificates required to get logs.

Fixes #2793
Closes #2574 (since this is no longer needed)

@vglafirov @aku105

[jackfrancis]

@sozercan see this PR:

https://github.com/Azure/acs-engine/pull/2915/files

Because this is a "bool" type we can't put a string in there (which is what PopulateClassicModeDefaultValue does)

[sozercan]

@jackfrancis thanks! updated

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@6457e15). Click here to learn what that means.
The diff coverage is 16.66%.

@@            Coverage Diff            @@
##             master    #2903   +/-   ##
=========================================
  Coverage          ?   49.74%           
=========================================
  Files             ?       91           
  Lines             ?    13932           
  Branches          ?        0           
=========================================
  Hits              ?     6930           
  Misses            ?     6359           
  Partials          ?      643

Impacted Files	Coverage Δ
pkg/acsengine/engine.go	63.95% <16.66%> (ø)

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[sozercan]

@jackfrancis let me know if its preferred to reuse an existing cert from /etc/kubernetes/cert instead of generating new ones for VK.

[jackfrancis]

@sozercan I think a dedicated cert is a fine first start, we can determine in practice if it's overkill. Is this PR a working prototype at this point?

[jackfrancis]

/approve
/lgtm

pending e2e

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