Improve performance of the Azure cloud provider

[DavidLangworthy]

Previously, the cloud provider called Get to obtain the status for each node pool. When running with a large number of node pools, e.g. 10+, these will cause Azure to throttle, not only the autoscaler, but all services operating under the same identity.

This commit replaces the Get call for each node pool with a List call which obtains the status for all the node pools at once. The frequency of these calls is set with the vmssSizeRefreshPeriod constant.

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[DavidLangworthy]

/assign @feiskyer

[feiskyer]

@DavidLangworthy thanks for the improvement. LGTM. could you follow https://git.k8s.io/community/CLA.md#the-contributor-license-agreement sign the CLA?

[feiskyer]

/area provider/azure

[feiskyer]

/approve

there's still a unit error, could you fix it?

[DavidLangworthy]

@feiskyer I have updated the expected values and am passing all tests but one. This is TestDeleteNodes. It overwrites the FakeStore. How should I deal with this? Also, are the expected values changes alright?

[mboersma]

@feiskyer tests are passing now, PTAL when time permits.

[feiskyer]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: feiskyer

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[feiskyer]

@DavidLangworthy would you like to cherry pick to stable branches?

[DavidLangworthy]

@feiskyer Yes, I would like this in 1.16 and 1.15. Should I just reissue this PR against those branches?