To increase the chances that job_dir and git_dir are on the
same filesystem in the default configuration, set the default
job_dir to /var/lib/zuul/builds.
Also, all zuul container images specify a volume at /var/lib/zuul,
therefore the docker-compose file does not need to specify the
same for the scheduler container.
Due to this, we were inadvertently running the executor with the
git_dir and job_dir on different filesystems in the quick-start.
That should no longer be the case.