add job source to the "Processing" log line json

test plan: run jobs, the "Processing" log line when the job starts
should include the source field among the other json fields. This source
field has the marginalia information about the web request or other job
that created this job.

Change-Id: If58da146b241b12093fd31822b93924481434143
Reviewed-on: https://gerrit.instructure.com/40352
Reviewed-by: Hubert depesz Lubaczewski <hlubaczewski@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>