[MM] NestedObject flatten

If a NestedObject only contains one field, the reviewers from terraform would require us to flatten the structure. For example, on behalf of cli, we have an `api.yml` saying
``` yaml
- !ruby/object:Api::Type::NestedObject
name: 'identity'
description: ''
required: false
azure_sdk_references: ['/identity']
properties:
- !ruby/object:Api::Type::Enum
name: 'type'
description: 'The type of Managed Identity used by the DiskEncryptionSet. Only SystemAssigned is supported.'
required: false
azure_sdk_references: ['/identity/type']
values:
- :SystemAssigned
default_value: :SystemAssigned
```
This should be replaced by
``` yaml
- !ruby/object:Api::Type::Enum
name: 'identityType'
description: 'The type of Managed Identity used by the DiskEncryptionSet. Only SystemAssigned is supported.'
required: false
azure_sdk_references: ['/identity/type']
values:
- :SystemAssigned
default_value: :SystemAssigned
```
This issue was initially filed to MM [here](https://github.com/VSChina/magic-modules/issues/37)