We've had a long-standing request from a customer to allow the

settings_local.py template to be customizable via a template. This

change enables this through a --settings-local-template parameter,

which can point to a local file that will be processed and turned into

the resulting settings_local.py.
The help output displays the default template file we use, so that

consumers can better find something to base theirs off of.
Other templates are not (at this time) customizable, though we could now

more easily do this in the future. Most of our templates are for web

servers or cron, rather than part of the Review Board runtime, and are

therefore less critical for the site installation.

Tested with and without a custom template. Saw the expected result in

each case.