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Convert ComputerName to lower case

**What this PR does / why we need it**:

When instances count is greater than 10, azure will generate instance hostname with upper cases. But kubelet always converts hostname to lower case.

So azure cloud provider should also covert OsProfile.ComputerName to lower case.

**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 #58274

**Special notes for your reviewer**:

This PR also adds more unit tests for scale sets.

**Release note**:

```release-note
NONE
```