Include "end_timestamp" attribute on workflow completion

When a Mistral workflow completes, include "end_timestamp" attribute in the action execution update request body.

We have a time util @ https://github.com/StackStorm/st2/blob/master/st2common/st2common/util/isotime.py.   It adds timezone awareness.  Example of usages @ https://github.com/StackStorm/st2/blob/master/st2common/st2common/services/action.py#L73.  Please make sure all end_timestamps are consistent with the start_timestamp.  Thanks.