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Reduce API calls for Azure instance metadata
**What this PR does / why we need it**:
Azure cloud provider gets a lot of `"Too many requests"` error when getting availability zones, instance types and node addresses. Hence kubelet won't be able to initialize itself sometimes.
This PR reduces such calls and alos switches to json API which is more stable.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes https://github.com/Azure/acs-engine/issues/3681
**Special notes for your reviewer**:
**Release note**:
```release-note
Reduce API calls for Azure instance metadata.
```
cc @ritazh @khenidak @andyzhangx
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