az deployment validate/create command fails with "too many 500 error responses"

**Describe the bug**
We are trying to create a new VM using a custom ARM template.

**az deployment validate/create** command consistently fails with this error:
````
Error occurred in request., RetryError: HTTPSConnectionPool(host='management.azure.com', port=443): Max retries exceeded with url: /subscriptions/e418758e-08c0-4562-b132-942b5a42fe2c/providers/Microsoft.Resources/deployments/aks-base-image/validate?api-version=2020-06-01 (Caused by ResponseError('too many 500 error responses',))
````

**To Reproduce**
Download this ARM template and save it as aks-base-image.txt
[aks-base-image.txt](https://github.com/Azure/azure-cli/files/5470147/aks-base-image.txt)

run these commands:
````
az deployment validate --location westus2 --template-file .\aks-base-image.txt
````
````
az deployment create --location westus2 --template-file .\aks-base-image.txt
````

Both commands will consistently fail with RetryError.

Note that if I go into Azure portal and deploy this ARM template using "Deploy a custom template", this succeeds, so this is not a template issue, but rather az cli issue.

**Expected behavior**
VM Deployment should succeed.

**Environment summary**
Windows, running azure cli on PowerShell.

**Additional context**
Output from az version:

{
"azure-cli": "2.11.1",
"azure-cli-core": "2.11.1",
"azure-cli-telemetry": "1.0.5",
"extensions": {
"azure-devops": "0.14.0"
}
}