Add job lifecycle logging

Various	points (e.g. created, running, processing events), are
structured into	json format and	output to /var/log/tower/job_lifecycle.log

As part	of this	work, the DependencyGraph is reworked to return
which job object is doing the blocking, rather than a boolean.

requirements/requirements.in

@@ -28,6 +28,7 @@ djangorestframework-yaml
 GitPython>=3.1.1  # minimum to fix https://github.com/ansible/awx/issues/6119
 irc
 jinja2>=2.11.0  # required for ChainableUndefined
+JSON-log-formatter
 jsonschema
 Markdown  # used for formatting API help
 openshift>=0.11.0  # minimum version to pull in new pyyaml for CVE-2017-18342