'az ml computetarget create aks' should support v2 and v3 GPU VM families

**Is your feature request related to a problem? Please describe.**
I'm unable to create an AKS cluster with the `az ml computetarget create aks` command using node types from v2 or v3 GPU families, such as `Standard_NC6s_v3`.
According to [AKS ARM provider reference](https://docs.microsoft.com/en-us/azure/templates/microsoft.containerservice/2018-03-31/managedclusters#managedclusteragentpoolprofile-object), AKS supports NC v2 and v3 machines. Yet, when I try to create the cluster using the Azure ML extension, I receive a parameter validation error claiming that only v1 NC machines are supported:
```
> az ml computetarget create aks --name myservice-v100 --agent-count 2 --vm-size Standard_NC6s_v3
Provisioning compute resources...
Resource creation submitted successfully.
Provisioning operation finished, operation "Failed"
Async operation failed with
StatusCode: 400
Message: The request is invalid
Compute object has provisioning state "Failed"
and provisioning errors: [{'code': 'BadRequest', 'message': 'The request is invalid', 'error': {'code': 'BadRequest', 'statusCode': 400, 'message': 'The request is invalid', 'details': [{'code': 'The request is invalid', 'message': "RequestId: 7293df66-0e58-4df7-a0d9-9dc96c544673\nError code: 'InvalidTemplate'. Target: ''. Message: 'Deployment template validation failed: 'The provided value 'Standard_NC6s_v3' for the template parameter 'agentVMSize' at line '1' and column '972' is not valid. The parameter value is not part of the allowed value(s): 'Standard_A10,Standard_A11,Standard_A2,Standard_A2_v2,Standard_A2m_v2,Standard_A3,Standard_A4,Standard_A4_v2,Standard_A4m_v2,Standard_A5,Standard_A6,Standard_A7,Standard_A8,Standard_A8_v2,Standard_A8m_v2,Standard_A9,Standard_D11,Standard_D11_v2,Standard_D12,Standard_D12_v2,Standard_D13,Standard_D13_v2,Standard_D14,Standard_D14_v2,Standard_D15_v2,Standard_D2,Standard_D2_v2,Standard_D3,Standard_D3_v2,Standard_D4,Standard_D4_v2,Standard_D5_v2,Standard_DS11,Standard_DS11_v2,Standard_DS12,Standard_DS12_v2,Standard_DS13,Standard_DS13_v2,Standard_DS14,Standard_DS14_v2,Standard_DS15_v2,Standard_DS2,Standard_DS2_v2,Standard_DS3,Standard_DS3_v2,Standard_DS4,Standard_DS4_v2,Standard_DS5_v2,Standard_F16,Standard_F16s,Standard_F2,Standard_F2s,Standard_F4,Standard_F4s,Standard_F8,Standard_F8s,Standard_G1,Standard_G2,Standard_NC6,Standard_NC12,Standard_NC24,Standard_NC24r,Standard_NV6,Standard_NV12,Standard_NV24,Standard_B1s,Standard_B1ms,Standard_B2s,Standard_B2ms,Standard_B4ms,Standard_B8ms'.'.'\n"}]}}]
```

**Describe the solution you'd like**
`az ml computetarget create aks` should support all GPU node types which are supported by AKS.

**Describe alternatives you've considered**
I can probably create the AKS cluster myself and attach it to the ML Workspace using `az ml computetarget attach aks`, but it will be much more complicated than simply creating the compute target with one command.