Add configurable description, URL and maintainer to fpm-generated package, set --conflicts on fpm

I don't think everyone will want their packages to have the maintainer set as Opscode, Inc. This very simple change allows it to default to what was hardcoded before, but lets individual package builders specify their own values for description, URL and maintainer.

Also it's nice to be able to have an Omnibus package conflict with another supplied package, to prevent confusion between a non-Omnibus and an Omnibus package.