Add config.json.sample instead of config.json

Configuration of Sensu may take place exclusively using the `conf.d` directory.
The addition of the original `config.json` file will occur upon upgrading the
Sensu omnibus package after a systems administrator had intentionally deleted
it. The restart of Sensu services will merge the stock configuration found in
the `config.json` with other configuration. This is not desired and the manual
removal of `config.json` is inconvenient after upgrade. This change ensures
that the package does not place a configuration file that would be loaded by
default. This change requires a documentation update to explain the possible
adoption of `config.json.sample` for new installations.

This change was not tested because of the high overhead of the build process. It would be much appreciated if the package maintainer would test this minor change on my behalf. Thank you.