Current event listener implementation does not handle network problems
well. E.g. if ssh stream connection is lost, it would not reconnect or
recover on its own. Instead of fixing the implementation, use
well-tested gerrit listener used by zuul. Explicitly specify version of
zuul to be 2.1.0 to avoid accidental breakages due to changes in zuul
Downside is that we need to install zuul and its dependencies just to
use gerrit listener.
parse_json_event function had to be changed, because zuul gerrit event
listener provides object, not json string. We still need to create
event from json in populate_db.py, so that part of the function has
been moved there.