[MM] Inconsistency in property types

I am generating `api.yaml` file using `autorest.cli` with input file: `azure-rest-api-specs\specification\compute\resource-manager\readme.md`

# Tag in use
`package-2019-07`.

# Cli section added in `readme.md`

``` yaml $(cli)
cli:
cli-name: compute
azure-arm: true
license-header: MICROSOFT_MIT_NO_VERSION
payload-flattening-threshold: 2
namespace: azure.mgmt.compute
package-name: azure-mgmt-compute
clear-output-folder: false
debug: true
disable-azure-cli: true
```

# Problem encountered

Struct `VirtualMachineProperties`, its field `AvailabilitySet` and `VirtualMachineScaleSet` both are of type `*SubResource`, but they are generated as different types in `computevirtualmachine/api.yaml`:
```
- !ruby/object:Api::Type::NestedObject
name: 'availabilitySet'
description: 'Specifies information about the availability set that the virtual machine should be assigned to. Virtual machines specified in the same availability set are allocated to different nodes to maximize availability. For more information about availability sets, see [Manage the availability of virtual machines](https://docs.microsoft.com/azure/virtual-machines/virtual-machines-windows-manage-availability?toc=%2fazure%2fvirtual-machines%2fwindows%2ftoc.json).

For more information on Azure planned maintenance, see [Planned maintenance for virtual machines in Azure](https://docs.microsoft.com/azure/virtual-machines/virtual-machines-windows-planned-maintenance?toc=%2fazure%2fvirtual-machines%2fwindows%2ftoc.json)

Currently, a VM can only be added to availability set at creation time. An existing VM cannot be added to an availability set.

This property cannot exist along with a non-null properties.virtualMachineScaleSet reference.'
required: false
azure_sdk_references: ['/availabilitySet', '/properties/availabilitySet']
properties:
- !ruby/object:Api::Azure::Type::ResourceReference
name: 'id'
description: 'Resource Id'
required: false
sample_value: /subscriptions/{{ subscription_id }}/resourceGroups/{{ resource_group }}/providers/Microsoft.Compute/availabilitySets/{{ name }}
resource_type_name: TBD
azure_sdk_references: ['/availabilitySet/id', '/properties/availabilitySet/id']
- !ruby/object:Api::Type::NestedObject
name: 'virtualMachineScaleSet'
description: 'Specifies information about the virtual machine scale set that the virtual machine should be assigned to. Virtual machines specified in the same virtual machine scale set are allocated to different nodes to maximize availability. Currently, a VM can only be added to virtual machine scale set at creation time. An existing VM cannot be added to a virtual machine scale set.

This property cannot exist along with a non-null properties.availabilitySet reference.

Minimum api‐version: 2019‐03‐01'
required: false
azure_sdk_references: ['/virtualMachineScaleSet', '/properties/virtualMachineScaleSet']
properties:
- !ruby/object:Api::Type::String
name: 'id'
description: 'Resource Id'
required: false
azure_sdk_references: ['/virtualMachineScaleSet/id', '/properties/virtualMachineScaleSet/id']
```
One of them is `String`, the other is `ResourceReference`.
Since these kind of fields usually need to be flattened, String type is more preferred.