fix bug 1525614: use mozlog for gunicorn

[willkg]

This sets gunicorn up with a config file to configure logging. It uses
existing webapp environment variables and mimics the way the webapp sets
up logging so that a local dev environment gets the console and server
environments get mozlog.

[willkg]

This works locally when doing "make run" just fine. That covers the "local dev environment" mode.

The "server" mode is a little harder to trigger. We have to do something like this:

$ docker-compose run --service-ports webapp shell
app@de9fded47e40:/app$ cd webapp-django/
app@de9fded47e40:/app/webapp-django$ ./manage.py collectstatic
...
app@de9fded47e40:/app/webapp-django$ cd ..
app@de9fded47e40:/app$ LOCAL_DEV_ENV= ./docker/run_webapp.sh 

Then both the webapp and gunicorn will log in mozlog format and you'll see stuff like this:

{"Timestamp": 1549907106113734400, "Type": "gunicorn.error", "Logger": "socorro", "Hostname": "de9fded47e40", "EnvVersion": "2.0", "Severity": 6, "Pid": 601, "Fields": {"host_id": "de9fded47e40", "msg": "Booting worker with pid: 601"}}
{"Timestamp": 1549907115202945536, "Type": "gunicorn.access", "Logger": "socorro", "Hostname": "de9fded47e40", "EnvVersion": "2.0", "Severity": 6, "Pid": 601, "Fields": {"host_id": "de9fded47e40", "msg": "172.18.0.1 - - [11/Feb/2019:17:45:15 +0000] \"GET / HTTP/1.1\" 200 6990 \"-\" \"Mozilla/5.0 (X11; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0\""}}
{"Timestamp": 1549907115305317888, "Type": "gunicorn.access", "Logger": "socorro", "Hostname": "de9fded47e40", "EnvVersion": "2.0", "Severity": 6, "Pid": 601, "Fields": {"host_id": "de9fded47e40", "msg": "172.18.0.1 - - [11/Feb/2019:17:45:15 +0000] \"GET /static/jquery/dist/jquery.js HTTP/1.1\" 304 0 \"http://localhost:8000/\" \"Mozilla/5.0 (X11; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0\""}}

I'm going to land this and verify it works on stage. self-r+