create webapp (kind: functionapp) with vnetName set but cant't get vnetName from configuration API

### problem
create webapp(kind: functionapp) with vnetName configed. Use config API ([getConfig](https://docs.microsoft.com/en-us/rest/api/appservice/webapps/getconfiguration)) fetch configurations, vnetName always returned as empty string.

### api-verision
2018-02-01
2019-08-01

### test steps

1. create webapp. [createOrUpdate](https://docs.microsoft.com/en-us/rest/api/appservice/webapps/createorupdate)
request data:
```json
{
"kind": "functionapp",
"location": "westus",
"properties": {
"enabled": true,
"serverFarmId": "/xxxxx/xxxx/xxx/xxx",
"siteConfig": {
"alwaysOn": false,
"appSettings": [
{
"name": "AzureWebJobsStorage",
"value": "xxxx"
},
{
"name": "FUNCTIONS_EXTENSION_VERSION",
"value": "~1"
},
{
"name": "AzureWebJobsDashboard",
"value": "xxxx"
}
],
"cors": {},
"http20Enabled": false,
"linuxFxVersion": "",
"use32BitWorkerProcess": true,
"vnetName": "ugx4kud60wf",
"webSocketsEnabled": false
},
"clientAffinityEnabled": false,
"httpsOnly": false
},
"tags": {}
}
```

2. use configuration api get webapp config ([getConfig](https://docs.microsoft.com/en-us/rest/api/appservice/webapps/getconfiguration))
return data:
```json
{
"type": "Microsoft.Web/sites/config",
"location": "West US",
"tags": {},
"properties": {

"limits": null,
"autoHealEnabled": false,
"autoHealRules": null,
"tracingOptions": null,
"vnetName": "",
"siteAuthEnabled": false,

"scmIpSecurityRestrictions": [
{
"ipAddress": "Any",
"action": "Allow",
"priority": 1,
"name": "Allow all",
"description": "Allow all access"
}
],
}
}
```

Start with the createOrUpdate and getConfiguration API definitions linked in the issue, comparing siteConfig.vnetName in API versions 2018-02-01 and 2019-08-01. Reproduce the request and response shown; done means a configured vnetName is returned by the configuration API instead of an empty string.

Written by the indexing model from the issue text.