Evalute if WEBSITE_DEFAULT_HOSTNAME should be used for setting hostname

`WEBSITE_DEFAULT_HOSTNAME` was introduced as part of the work to address regional domain name changes to app service. Here is a sample value of this new app setting. `test-h4g6apbtgfcdhef3.eastus.azurewebsites.net`

Currently we use the WAS_DEFAULT_HOSTNAME header injected by the front end as a backup for identifying the correct hostname. https://github.com/Azure/azure-functions-host/blob/6c23426d7de334e628212eba095002f7d8a78ffe/src/WebJobs.Script/HostNameProvider.cs#L53-L62

We should evaluate if we should switch to using `WEBSITE_DEFAULT_HOSTNAME`