Fix InstanceActionEvent traceback parameter not serializable

The change to use InstanceActionEvent in compute.utils.EventReporter
changed the order of how things are done. Before, traceback info were
converted to strings before being sent to the conductor. Now, since
the object method being used remotes itself, the order becomes the
opposite and any captured tracebacks are sent as is, resulting in
errors during messaging.

Change-Id: Ibedddd606b9d25ffbbb2b1b5358cf0aa4741083f
Closes-Bug: #1317804