Cannot retrieve the master key using the Key management API

Hi I have a Function V2 that I've just created and I am trying to retrieve the `_master` key as described in the [Key management API](https://github.com/Azure/azure-functions-host/wiki/Key-management-API#host-key-resource-adminhostkeyskeyname) documentation.

> The host key resource exposes the same APIs as the function keys collection resource.
**Note**: The master key is exposed under the reserved name _master and DELETE is not supported

Function Get-Function-Jwt {
Param (
[string] [Parameter(Mandatory = $true)] $resourceGroupName,
[string] [Parameter(Mandatory = $true)] $functionAppName
)

$resourceType = "Microsoft.Web/sites/config"
$resourceName = "$functionAppName/publishingcredentials"
$publishingCredentials = Invoke-AzureRmResourceAction -ResourceGroupName $resourceGroupName -ResourceType $resourceType -ResourceName $resourceName -Action list -ApiVersion 2015-08-01 -Force

$accessToken = [Convert]::ToBase64String([Text.Encoding]::ASCII.GetBytes(("{0}:{1}" -f $publishingCredentials.Properties.PublishingUserName, $publishingCredentials.Properties.PublishingPassword)))

$jwt = Invoke-RestMethod -Uri "https://$functionAppName.scm.azurewebsites.net/api/functions/admin/token" -Headers @{Authorization = ("Basic {0}" -f $accessToken)} -Method GET

return $jwt
}

Function Get-HostKey {
Param (
[string] [Parameter(Mandatory = $true)] $functionAppName,
[string] [Parameter(Mandatory = $true)] $hostKeyName,
[string] [Parameter(Mandatory = $true)] $jwt
)

return (Invoke-RestMethod -Uri "https://$functionAppName.azurewebsites.net/admin/host/keys/$hostKeyName" -Headers @{Authorization = ("Bearer {0}" -f $jwt)} -Method Get).value
}

# Connect-AzureRmAccount
$resourceGroupName = ""
$functionAppName = "function app name"

$jwt = Get-Function-Jwt -resourceGroupName $resourceGroupName -functionAppName $functionAppName
Get-HostKey -functionAppName $functionAppName -hostKeyName "_master" -jwt $jwt

Write-Host $hostKey

Here is the exception:

> Invoke-RestMethod : The remote server returned an error: (404) Not Found.

If I try with the `default` host key, it is working fine.

Also I've tried the approach describe [here](https://github.com/Azure/azure-functions-host/issues/3378)

But I have another exception containing a link to this documentation:

- [Changes to Key Management in Functions V2](https://github.com/Azure/azure-functions-host/wiki/Changes-to-Key-Management-in-Functions-V2)

I don't really want to change the `AzureWebJobsSecretStorageType` to `files` so at the moment I am kind of stuck.

Start with the Key management API and Changes to Key Management in Functions V2 documentation, then trace the /admin/host/keys/{keyName} endpoint and the AzureWebJobsSecretStorageType behavior. Compare the documented _master request with the working default-key request; done means the master-key behavior is either restored without changing storage type or clearly documented as unsupported.

Written by the indexing model from the issue text.