Quote keys in the generated logstash.conf

Certain field names are not allowed unquoted in the logstash config. As an example, this `json` filter definition produces an error during startup:

    json {
      @message => 'json_data'
      type => 'syslog'
    }

The error message is:

    Error at line 70, column 5: "@message => 'json_data'\n    type => 'syslog'\n  }\n\n"

This patch quotes the key names in the generated logstash config which allows logstash to parse the config and evaluate the filter as intended.