This adds a "job-graph" subcommand which uses the freeze-jobs api
to return information about what jobs may be run.
This also adds a new formatter, "dot". The only command supporting
this formatter is job-graph. To see it in action, try:
zuul-client --format dot --zuul-url https://zuul.opendev.org job-graph --tenant openstack --pipeline check --project opendev/system-config --branch master | xdot -
Finally, this also adds "json" as an acceptable alias to "JSON" when
specifying the output format since that is a widely used convention.
Change-Id: I9adc3ab87bfa11432ae621b65dd94189bb17e42c
f999949aed