HHVM: improvements to logging

* Set hhvm.debug.core_dump_report_directory to /var/log/hhvm, which
  will make it write stack traces to this directory.
* Update the Upstart job configuration to look for a stack trace in
  $HHVM_LOG_DIR rather than in /tmp.
* Configure rsyslog to write HHVM's error log to /var/log/hhvm/error.log.
* Set the owner of /var/log/hhvm to 'syslog', so rsyslog can write to it.
* Set the permissions of /var/log/hhvm to 0775, so that HHVM's GID allows
  it to write stack traces to that location.
* Set the permissions of /run/hhvm/cache to 0755 rather than 0750. MediaWiki's
  database passwords, etc. are in world-readable files on the app server; I
  don't think HHVM's PHP bytecode cache is an issue.
* Configure logrotate to retain 90 days of HHVM error logs, and 7 days of
  stack traces.
* The HHVM rsyslog filter does not discard the message, and its priority
  is lower than MediaWiki's rsyslog config, so MediaWiki is still
  able to capture HHVM log messages for fluorine.

Change-Id: I11b85c87a8902de28cba9f71840885a9864ec1df
Merge "HHVM: improvements to logging" into production