MediaWiki/HHVM: log fatals to fluorine:/a/mw-log/hhvm-fatal.log

* Remove hstr Python script. It's not useful at the moment. It can always be
  ressurected later.
* Set HHVM_TRACE_HANDLER to "/usr/bin/logger --tag hhvm-fatal --file ".
  This will log the fatal to syslog with the 'hhvm-fatal' tag.
* Configure MediaWiki to forward syslog messages tagged with 'hhvm-fatal' to
  fluorine.
* Rename syslog.pp's 'apache_log_aggregator' parameter to 'log_aggregator', to
  make it clear that it is not Apache-specific.

Change-Id: I98f9fc20cbe9c32ddee04a77427e06446e2f6de1