dns: _get_service_endpoint requires a context

In `_get_ptr_details`, `pyrax._get_service_endpoint` is being called without the "context" argument.  When interacting with `PTR` related actions such as `dns.list_ptr_records` this causes and exception to be raised:

```
  File "/Users/matt/python_venvs/ansibledev/lib/python2.7/site-packages/pyrax/clouddns.py", line 1298, in list_ptr_records
    return self._manager.list_ptr_records(device)
  File "/Users/matt/python_venvs/ansibledev/lib/python2.7/site-packages/pyrax/clouddns.py", line 915, in list_ptr_records
    href, svc_name = self._get_ptr_details(device, device_type)
  File "/Users/matt/python_venvs/ansibledev/lib/python2.7/site-packages/pyrax/clouddns.py", line 879, in _get_ptr_details
    ep = pyrax._get_service_endpoint("compute")
TypeError: _get_service_endpoint() takes at least 2 arguments (1 given)
```

This pull request adds the "context" parameter to the `_get_service_endpoint` calls.