Celery generally likes to own the world. Rather than just being a useful

library for talking to queues, it also wants to be the command

management tool, even taking over version and help information.
This is generally not a great experience. We end up exposing a bunch of

commands and options that aren't needed for Review Bot. We've wanted to

drop this for a long time. This change does that.
Now, we have a simple reviewbot command line tool that takes a handful

of vetted options. Most of these are Celery options, but only ones that

we want to pass on to Celery. It's otherwise under our control, and we

can build upon this in the future to take new options.
Once invoked, we set up a Celery instance, and a Worker command instance

(which is what powered the reviewbot command in the past). We treat

this somewhat like a utility API, calling the methods we need to set up

the right state.
There is a trade-off here. If Celery makes substantial changes to the

internals, we'll have to update a couple things. That's not a big deal,

though, as most of what we're bypassing is the argument parsing and

initial environment setup. Once we've tackled this ourselves, we just

call run() on the worker command.
The one annoying complication is the --detach argument. Normally,

Celery handles this completely for us, but not really if being invoked

programmatically. Which is sort of okay. When it detaches, it hard-codes

celery worker ..., which isn't what we wanted anyway.
The old reviewbot worker invocation is still supported, but emits a

deprecation message. We can remove this in future release.

Tested all options on Python 2 and 3, and verified that they invoked the

right behavior in Celery.