ARM template keyvault secret upload/push is not working

[Key Vault Secrets](https://github.com/Azure/azure-quickstart-templates/tree/master/101-key-vault-create)

### Issue Details

I am not sure what the problem is. I have a template that creates a few resources, then a key vault, then tries to save some of the output of the resources to the key vault as secrets. It is this secret save that was failing.

So I shortened the template to try to find the root cause, until I was left with just:

`keyvault.json`:
```
{
"$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
"contentVersion": "1.0.0.0",
"parameters": {
"kv_name": {
"type": "string"
}
},
"functions": [],
"resources": [
{
"type": "Microsoft.KeyVault/vaults/secrets",
"name": "[concat(parameters('kv_name'), '/', 'container_server')]",
"apiVersion": "2019-09-01",
"properties": {
"value": "whatever"
}
}
]
}
```

The key vault already exists. I have set the access policy to give me (my AD user ID) full authority for all secrets on the vault, and there is no network firewall, it is open.

Then I run, on the CLI:

az deployment group create -f ./keyvault.json -g

This is as bare-bones as I can imagine. Everything else is working, except for the secrets, which I have shrunk down to the template shown above.

I keep getting the following unfortunately useless error:

```
Deployment failed. Correlation ID: f4ccf19c-b6a1-4e46-9604-81d29044e957. {
"error": {
"code": "BadRequest",
"message": ""
}
}
```

Empty message, unhelpful "BadRequest" code. :(

Any ideas what is happening? There is no other possible security issue that could be blocking it. I am able to upload secrets myself via the portal. I am able to launch other services, so I think my IAM credentials should be fine.

Start with the minimal keyvault.json template and reproduce the failure using `az deployment group create -f ./keyvault.json -g `. Compare the deployment response with the Key Vault secret resource and its existing access-policy setup; done means the template creates the `container_server` secret and returns a useful successful deployment result.

Written by the indexing model from the issue text.