Use current AKS Ubuntu 18.04 distro name

[matmerr]

What this PR does / why we need it:
Distro name changed in aks-engine to aks-ubuntu-18.04 :
https://github.com/Azure/aks-engine/blob/master/docs/topics/clusterdefinitions.md#kubernetesconfig

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #
E2E Tests

Special notes for your reviewer:

Release note:

NONE

[codecov]

Codecov Report

Merging #393 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #393   +/-   ##
=======================================
  Coverage   42.59%   42.59%           
=======================================
  Files          26       26           
  Lines        3773     3773           
=======================================
  Hits         1607     1607           
  Misses       1902     1902           
  Partials      264      264

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[tamilmani1989]

lgtm