Fixes issues with CheckSkuAvailability operation

[felixwa]

1. Changes type of skuName property from Sku to SkuName.
2. Marks some fields as required.

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This change is 

[jianghaolu]

error: Found errors in validating the response with statusCode "200" for example in spec for operation "CognitiveServicesAccounts_GetProperties".:

{ code: 'RESPONSE_VALIDATION_ERROR',
  message: 'Found errors in validating the response with statusCode "200" for example in spec for operation "CognitiveServicesAccounts_GetProperties".',
  innerErrors: 
   [ { code: 'INVALID_RESPONSE_BODY',
       errors: 
        [ { code: 'INVALID_TYPE',
            params: [ 'object', 'string' ],
            message: 'Expected type object but found type string',
            path: [],
            description: 'Cognitive Services Account is an Azure resource representing the provisioned account, its type, location and SKU.' } ],
       message: 'Invalid body: Expected type object but found type string',
       path: [] } ] }

[jianghaolu]

error: Found errors in validating the response with statusCode "200" for x-ms-example "Check SKU Availability" in operation "CheckSkuAvailability_List".:

{ code: 'RESPONSE_VALIDATION_ERROR',
 message: 'Found errors in validating the response with statusCode "200" for x-ms-example "Check SKU Availability" in operation "CheckSkuAvailability_List".',
 innerErrors: 
  [ { code: 'INVALID_RESPONSE_BODY',
      errors: 
       [ { code: 'INVALID_TYPE',
           params: [ 'string', 'null' ],
           message: 'Expected type string but found type null',
           path: [ 'value', '0', 'message' ],
           description: 'Additional error message.' },
         { code: 'INVALID_TYPE',
           params: [ 'string', 'null' ],
           message: 'Expected type string but found type null',
           path: [ 'value', '0', 'reason' ],
           description: 'Reason why the SKU is not available.' } ],
      message: 'Invalid body: Value failed JSON Schema validation',
      path: [] } ] }

[jianghaolu]

Please fix the examples after the payload definition for Sku is changed.

[felixwa]

@jianghaolu ,
For issue 1: I cannot get it when run "node cli validate-example". What tool did you use?
For issue 2: it is a known issue, the field can be a string or null. I have consulted this issue with reviewer last time and she said this is a known issue. PR: #1090 Issue: #798

[jianghaolu]

Seems like both are false alarms. Otherwise looks good to me.

[AutorestCI]

No modification for Ruby

[AutorestCI]

No modification for NodeJS

[AutorestCI]

Azure/azure-sdk-for-python#1175