Add --config-files so etc files don't get overwritten

Fixes mesosphere/mesos-deb-packaging#3

NOTE: The packaged conffiles doesn't use absolute paths and will fail lintian tests. jordansissel/fpm@1dfbdfdd8d75164286a50a2caacc42e9c2e140a0 of fpm fixes this, but it is not available on RubyGems yet.