Adding onlyif, unless and check_cmd support to file.managed

Adding onlyif, unless and check_cmd support to file.managed state.  onlyif, run the named command only if the command passed to the onlyif option.  unless, only run the named command if the command passed to the unless option returns false.  check_cmd, run the state only if the check_cmd succeeds, can be used to verify a configuration file is in the correct state before putting it in place.  Per #12818