Updates from aks-engine spike

[jackfrancis]

What this PR does / why we need it: During our experimental spike to aks-engine, several forward-looking functional changes were made that we want here. From the commit history of the branch that this PR derives from:

• Fix prow set up
• e2e changes
• removing openshift artifacts
• accelerated networking rationalization, with tests
• remove additional sed statements for ip-masq addons
• Update go-dev tools image for go 1.11.2
• remove unused azconst methods
• add support PB6 vm skus
• update azure_const unit test
• update tiller versions in the recent versions of kubernetes
• VSTS VHD pipeline hosted ubuntu pool
• azureconst cruft
• scale: persist scale down in api model
• Add support for Kubernetes 1.11.5
• Fix docker-engine install in VHD pipeline
• remove IsOpenShift from E2E

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

Special notes for your reviewer:

If applicable:

• [ ] documentation
• [ ] unit tests
• [ ] tested backward compatibility (ie. deploy with previous version, upgrade with this branch)

Release note:

Updates from aks-engine spike

[codecov[bot]]

Codecov Report

Merging #4302 into master will increase coverage by 4.93%. The diff coverage is 80.64%.

@@            Coverage Diff             @@
##           master    #4302      +/-   ##
==========================================
+ Coverage   50.49%   55.43%   +4.93%     
==========================================
  Files         109      109              
  Lines       17158    16050    -1108     
==========================================
+ Hits         8664     8897     +233     
+ Misses       7710     6369    -1341     
  Partials      784      784

[tariq1890]

/lgtm pending e2e