engine : User is UNKNOWN in async task completion events

In async tasks after the user has been logged out the
audit log messages show user as UNKNOWN.

This patch uses the current user from parameters to get the
user who initiated the task if the user is not logged in.

Change-Id: Ic0904f9f6d9432ecac1cebee3790eae902a1329a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=869660
Signed-off-by: Ravi Nori <rnori@redhat.com>