Add an optional config file for the Review Bot worker.

Review Request #8773 — Created Feb. 21, 2017 and submitted — Latest diff uploaded




The worker currently runs without configuration, but some of the new features
we're looking at will require it. In particular, the immediate need is that
PMD, a tool which we'll be adding, does not have a standard installation
mechanism, and often is run via a script from the download directory. The path
to this script has to be configured for the tool to work.

While I was in here, I fixed up the task serialization/accept config to get rid
of a big warning about accepting pickle content (which is insecure and going
away in future versions), and corrected the method of setting the broker URL,
which I hadn't noticed was incorrect because it defaults to localhost.

Ran the worker with and without a config file in place. Saw the expected
output, and that the (future) PMD tool loaded correctly.