Add pipeline source

A TriggerEvent may originate from a trigger that does not represent
the canonical location of the project source.  For instance, the
timer trigger strangely depends on the gerrit trigger to actually
handle Git operations behind the scenes.  Instead, make an explicit
association between pipelines and their source triggers so that
their event trigger does not need to have that implicit association.

This is a step toward having pipelines support multiple triggers
(they already support multiple reporters).

Change-Id: Ie80ffde411fe40fddfc4496b7adb0004f660c48c
Merge "Add pipeline source"