[Compute] Fix List VM Extension API name, description updates and other fixes.

[hyonholee]

• Fix the name of List VM Extension API name, which was set wrong.
• 2018-06-01 version was missing the previous fixes for the response of POST APIs.
• Minor description updates.
• Add VMSS Ip Tag

This checklist is used to make sure that common issues in a pull request are addressed. This will expedite the process of getting your pull request merged and avoid extra work on your part to fix issues discovered during the review process.

PR information

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• If applicable, the PR references the bug/issue that it fixes.
• Swagger files are correctly named (e.g. the api-version in the path should match the api-version in the spec).

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• My spec meets the review criteria:
  • The spec conforms to the Swagger 2.0 specification.
  • The spec follows the guidelines described in the Swagger checklist document.
  • Validation tools were run on swagger spec(s) and have all been fixed in this PR.

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[sarangan12]

Why are we removing the schema? If this is removed, what will be schema of the response?

[hyonholee]

For Delete operation, the body of response is empty. So the Swagger spec is wrong, and we get nothing for OperationStatusResponse anyway. So I removed it.

https://github.com/Azure/azure-sdk-for-net/blob/6a493f405a59f7ea4f984143d5afe39ddeb8a8dd/src/SDKs/Compute/Compute.Tests/SessionRecords/Compute.Tests.AvailabilitySetTests/TestOperations.json#L747

[lmazuel]

@AutorestCI regenerate azure-sdk-for-python