Add an optional Review Bot worker config file, use for PMD.

Review Request #8769 — Created Feb. 21, 2017 and discarded — Latest diff uploaded




I had written the PMD tool for Review Bot to configure the path to the
executable via the UI, but that approach is a little problematic in the case
where there are multiple workers running. In that case, a worker that didn't
have a valid PMD installation would take the task off the queue and then report
an error (even if there was a worker that could handle it).

I've added an optional config file for the worker, which can be used to
configure the path. If the path is unconfigured, or misconfigured, the worker
will not listen for PMD changes.

This config file will be used for more stuff in the future.

Ran the worker and saw that it loaded the config file correctly.