Add enqueue_time to JSON output for a change

This adds a javascript timestamp for each change in the JSON output.

Change-Id: I7965c6a7ff6e4e5522da3c51936cfdfb6cf36e91
Reviewed-on: https://review.openstack.org/32064
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins