StorageProfile default is now ManagedDisks

[jackfrancis]

What this PR does / why we need it: default to ManagedDisks for storage, i.e. Microsoft.Compute/availabilitySets, instead of StorageAccount, i.e., Microsoft.Storage/storageAccounts

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #2799

Special notes for your reviewer: Also rationalized examples/ api models to remove any perception that ManagedDisks is an explicit opt-in feature.

If applicable:

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

Release note:

StorageProfile default is now ManagedDisks

[jackfrancis]

@khenidak @lachie83 FYI

[codecov]

Codecov Report

Merging #2833 into master will decrease coverage by 1.71%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2833      +/-   ##
==========================================
- Coverage   48.99%   47.28%   -1.72%     
==========================================
  Files          91       86       -5     
  Lines       13788    12815     -973     
==========================================
- Hits         6755     6059     -696     
+ Misses       6409     6197     -212     
+ Partials      624      559      -65

Impacted Files	Coverage Δ
pkg/acsengine/defaults.go	72.25% <100%> (+7.44%)	⬆️
cmd/generate.go	26.43% <0%> (-5.95%)	⬇️
cmd/deploy.go	23.8% <0%> (-1.78%)	⬇️
pkg/api/vlabs/validate.go	42.93% <0%> (-1.57%)	⬇️
pkg/acsengine/engine.go	62.86% <0%> (-1.51%)	⬇️
pkg/operations/kubernetesupgrade/upgrader.go	56.66% <0%> (-0.84%)	⬇️
pkg/api/converterfromagentpoolonlyapi.go	22.08% <0%> (-0.42%)	⬇️
pkg/acsengine/transform/transform.go	59.81% <0%> (-0.19%)	⬇️
pkg/api/common/versions.go	82.02% <0%> (-0.11%)	⬇️
pkg/acsengine/defaults-kubelet.go	93.5% <0%> (-0.09%)	⬇️
... and 15 more

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[CecileRobertMichon]

lgtm