Adjust logstash.filters configuration

This includes a few changes initiated by logsearch/logsearch-boshrelease#58...

 * by default, embedded filters are no longer included if you specify your own filters (`example/warden.yml` provides an example to reuse them within a deployment)
 * this adds support for `file://*` which will include `*.conf` files from the specified directory
 * this simplifies the config building a little bit and allows `logstash.filters` to be either a string or array of location paths (if an array, files of the same name will overwrite earlier extractions)