fix for issue #14655

Fix for issue #14655

timedelta.microseconds returns the microseconds portion of the timestamp. Instead we call total_seconds() and the multiple by 1000.

```
>>> start_time = datetime.datetime.now()
>>> finish_time = datetime.datetime.now()
>>> (finish_time-start_time).total_seconds() * 1000
14567.464
>>>
```
Python 2.6 doesn't have this it turns out :(

How about:

```python
delta = finish_time - start_time
ret['duration'] = delta.seconds * 1000 + delta.microseconds
```

This ends up with it as an float, not an integer. Salt folks: does this change matter?