feat: enable re-use of VHD

[jackfrancis]

Reason for Change:

This PR re-orders the "do I have the 'created by AKS VHD CI' OS signature?" and the "does my cluster configuration explicitly declare a VHD distro?" checks in order to enable AKS VHD re-use scenarios. E.g.:

• create a new managed image / shared gallery image / VHD from the AKS VHD

Specifically, the boolean variable FULL_INSTALL_REQUIRED in our CSE implementation is functionally coupled with the VHD CI implementation that pairs with that CSE implementation at that commit of aks-engine. In practice, this means that for folks who want to re-use the AKS VHD as a base for creating their own Kubernetes node OS images can (1) take advantage of the optimized CSE code paths (e.g., don't execute unnecessary apt operations) and (2) implement the proper cleanup functionality so that the resultant Kubernetes node isn't unnecessarily burdened with the storage overhead of dozens of unused, pre-pulled Kubernetes hyperkube images (we use a common VHD to support a large set of Kubernetes versions).

Issue Fixed:

Fixes #2036

Requirements:

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

Notes:

[jackfrancis]

@CecileRobertMichon I think this might fix #2036

[CecileRobertMichon]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [CecileRobertMichon,jackfrancis]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

Merging #2130 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2130   +/-   ##
=======================================
  Coverage   76.63%   76.63%           
=======================================
  Files         135      135           
  Lines       20655    20655           
=======================================
  Hits        15829    15829           
  Misses       3903     3903           
  Partials      923      923