[Resources] Response from Azure Stack doesn't comply with the spec subscriptions.json

When using `2019-06-01` with Azure Stack to get subscriptions (in this case `management.redmond.azurestack.corp.microsoft.com`), the response doesn't comply with the REST spec [`subscriptions.json`](https://github.com/Azure/azure-rest-api-specs/blob/master/specification/resources/resource-manager/Microsoft.Resources/stable/2019-06-01/subscriptions.json). For example, `authorizationSource` and `managedByTenants` are missing from the response.

https://github.com/Azure/azure-rest-api-specs/blob/76b6bd1989a8248987219b840f525b02e07807dc/specification/resources/resource-manager/Microsoft.Resources/stable/2019-06-01/subscriptions.json#L271-L277

```
> az cloud register -n redmond --endpoint-resource-manager "https://management.redmond.azurestack.corp.microsoft.com/"
> az cloud set -n redmond
> az login --debug

msrest.http_logger : Request URL: 'https://management.redmond.azurestack.corp.microsoft.com/subscriptions?api-version=2019-06-01'
msrest.http_logger : Request method: 'GET'

msrest.http_logger : Response status: 200
msrest.http_logger : Response content:
msrest.http_logger :
{
"value": [
{
"offerId": "/delegatedProviders/default/offers/full-offer",
"id": "/subscriptions/c9b8dfff-aff0-49b5-93bd-a1581f1896a0",
"subscriptionId": "c9b8dfff-aff0-49b5-93bd-a1581f1896a0",
"tenantId": "81cfafcd-8b9a-4419-9c85-823b54076b55",
"displayName": "azclidevtools",
"state": "Enabled",
"subscriptionPolicies": {
"locationPlacementId": "AzureStack"
}
}
]
}
```

This causes failure in Azure CLI when [retrieving `managed_by_tenants`](https://github.com/Azure/azure-cli/blob/80c7bc79d416007d86848650b149d5fa24783c19/src/azure-cli-core/azure/cli/core/_profile.py#L275-L276):

```
File "C:\Users\jiasli\AppData\Local\Programs\Python\Python38\lib\site-packages\azure\cli\core\_profile.py", line 276, in _normalize_properties
subscription_dict[_MANAGED_BY_TENANTS] = [{_TENANT_ID: t.tenant_id} for t in s.managed_by_tenants]
TypeError: 'NoneType' object is not iterable
```

The response of the public `AzureCloud` is:

```
> az cloud set -n AzureCloud
> az login --debug

msrest.http_logger : Request URL: 'https://management.azure.com/subscriptions?api-version=2019-06-01'
msrest.http_logger : Request method: 'GET'

msrest.http_logger : Response status: 200
msrest.http_logger : Response content:
msrest.http_logger :
{
"value": [
{
"id": "/subscriptions/0b1f6471-1bf0-4dda-aec3-cb9272f09590",
"authorizationSource": "RoleBased",
"managedByTenants": [
{
"tenantId": "2f4a9838-26b7-47ee-be60-ccc1fdec5953"
}
],
"subscriptionId": "0b1f6471-1bf0-4dda-aec3-cb9272f09590",
"tenantId": "54826b22-38d6-4fb2-bad9-b7b93a3e9c5a",
"displayName": "AzureSDKTest",
"state": "Enabled",
"subscriptionPolicies": {
"locationPlacementId": "Internal_2014-09-01",
"quotaId": "Internal_2014-09-01",
"spendingLimit": "Off"
}
}
],
"count": {
"type": "Total",
"value": 1
}
}
```

If `2019-06-01` is not supported, the service should return "unsupported API version" instead a wrong response which doesn't comply with the spec.

Read specification/resources/resource-manager/Microsoft.Resources/stable/2019-06-01/subscriptions.json around lines 271-277 and compare its requirements with the captured Azure Stack response. Then inspect azure-cli-core/azure/cli/core/_profile.py around lines 275-276 to understand the reported failure; done means the service complies with the specification or explicitly rejects the unsupported API version.

Written by the indexing model from the issue text.