allow more flexible filter file names and grok patterns dir

[jxstanford]

Would be nice to be able to specify the placement of the patterns folder in the container. For example, in our environment, we put it in /logstash/patterns. In the current implementation, we would need to edit the filter files to point at /etc/logstash/patterns.

Similarly, would be nice to support flexible naming conventions for the filter files. In our environment, we've standardized on NNN-filter-purpose. An example would be 400-filter-host-syslog.