Delete request of the workspace in MachineLearningServices RP is async but not marked as long-running-operation in swagger

[ArcturusZhang]

Original issue: hashicorp/terraform-provider-azurerm#6696

Based on the log provided by a customer, the response header of a DELETE request of a workspace is

[DEBUG] AzureRM Response for https://management.azure.com/subscriptions/SUBSCRIPTION_ID/resourceGroups/pcamvp-dev123-rg/providers/Microsoft.MachineLearningServices/workspaces/pcamvp-dev123-aml?api-version=2019-11-01: 
HTTP/2.0 202 Accepted
Cache-Control: no-cache
Date: Fri, 01 May 2020 10:50:20 GMT
Expires: -1
Location: https://management.azure.com/subscriptions/SUBSCRIPTION_ID/providers/Microsoft.MachineLearningServices/locations/eastus/workspaceOperationsStatus/5efa8409-6789-46ef-8e53-fc095288636b?api-version=2018-03-01-preview
Pragma: no-cache
Request-Context: appId=cid-v1:2d2e8e63-272e-4b3c-8598-4ee570a0e70d
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Content-Type-Options: nosniff
X-Ms-Client-Request-Id: 5c126be6-c662-4994-8c4a-0bbaa058c314
X-Ms-Client-Session-Id: 
X-Ms-Correlation-Request-Id: 8c0b26d1-4d3a-2701-3a5f-f5476c235cc9
X-Ms-Ratelimit-Remaining-Subscription-Deletes: 14999
X-Ms-Request-Id: 16459393-90b8-4ffb-b765-16f21025c595
X-Ms-Routing-Request-Id: UKSOUTH:20200501T105020Z:16459393-90b8-4ffb-b765-16f21025c595
Content-Length: 0

The header Location indicates this request is an async operation, but the swagger is not marked x-ms-long-running-operation: true, and also 202 should also be added into the responses block.

[ghost]

Thanks for the feedback! We are routing this to the appropriate team for follow-up. cc @azureml-github.