Dedicate "triggered by" field in CLI

In the WebUI, we have a very handy column for seeing exactly what kicked off each execution:

This information is kind of available in the CLI, but you have to specify that you want to see the context attribute, and even then its buried in a bunch of JSON:

mierdin@stackstorm:~$ st2 execution list -a context
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| context                                                                                                                                                                                                                                      |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| {u'user': u'stanley'}                                                                                                                                                                                                                 |
| {u'trigger_instance': {u'id': u'59a4a21402ebd5583a131dfa', u'name': None}, u'trace_context': {u'id_': u'59a4a21402ebd5583a131dfb', u'trace_tag': u'st2.IntervalTimer-bdd470f4-0fbe-41a2-be90-112cf3d7754d'}, u'rule': {u'id':                |
| u'59a4a1f602ebd558f14a66dc', u'name': u'timer_rule'}, u'user': u'stanley'}                                                                                                                                                                   |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

It would be nice if this information was given a dedicated field so we could easily display it without taking up a bunch of screen space. It might be worth considering making this display by default, but that has other implications.

Start with the st2 execution list command and its existing -a context output. Determine how the trigger information should be represented as a compact dedicated field and whether it should appear by default. Done means users can identify what triggered each execution without parsing the full context JSON.