Include "end_timestamp" attribute on workflow completion

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

Yep, I was aware of that, but I wasn't sure if we want st2mistral to depend on st2common - I thought we don't want that.

Let me know if you think it's fine to use st2common here. I personally think current approach is better since st2common is not available as a standalone package on PyPi and depending on it here would introduce complications and coupling.

Current approach has exactly the same end result - `utcnow()` returns a UTC timestamp without timezone info. The API expects ISO 8601 formatted date string with a time zone info and we add UTC time zone suffix when generating a string so the format and end result is exactly the same.