logging: Remove logging.conf, use log_dir only
logging: Log to file, OpenStack to local3, ignore

All OpenStack services now log to /var/log/{service}. Log files in these
directories are read by rsyslog and forwarded to Logstash, if forwarding
is enabled. We also ignore local3 writing into /var/log/syslog due to
duplication.
Merge pull request #525 from blueboxgroup/logging-refactor

logging: Remove logging.conf, use log_dir only