logstash: Improve filters

* Parse type:apache2 messages without client ip.
* Parse type:apache2 PHP messages with multi-word severity level
* Extract exception_id, url, line, file and backtrace from
  type:exception
* Remove failed allocation size and function anchor tag noise from
  from normalized_message for fatalmonitor messages
* Trim fatalmonitor normalized_message to 255 character max length to
  work better with Elasticsearch term facets

Change-Id: I832f8bb63ca33f9be6496610e49a76d6aa23680e