`az network vnet-gateway vpn-client show-url` unhandled exception

## Describe the bug

**Command Name**
`az network vnet-gateway vpn-client show-url`

**Errors:**
```
'str' object has no attribute 'get'
Traceback (most recent call last):
python3.7/site-packages/knack/cli.py, ln 206, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 578, in execute
raise ex
cli/core/commands/__init__.py, ln 636, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 629, in _run_job
six.reraise(*sys.exc_info())
...
site-packages/msrestazure/polling/arm_polling.py, ln 262, in get_status_from_resource
status = self._get_provisioning_state(response)
site-packages/msrestazure/polling/arm_polling.py, ln 186, in _get_provisioning_state
return body.get("properties", {}).get("provisioningState")
AttributeError: 'str' object has no attribute 'get'
```

## To Reproduce:
Steps to reproduce the behavior:

- Create a VPN gateway, run `az network vpn-gateway vpn-client generate -g xxx -n yyy`
- Run `az network vnet-gateway vpn-client show-url -g xxx -n yyy`

## Expected Behavior
Don't fail with an unhandled exception.

## Environment Summary
```
Darwin-18.6.0-x86_64-i386-64bit
Python 3.7.4
Shell: bash

azure-cli 2.0.69 *

Extensions:
aks-preview 0.4.6

```
## Additional Context