Add log_dir and use by default instead of log_file

The log_file directive can end up forcing all agents (which are
called with --config-file /etc/quantum/quantum.conf) to log to the
/var/log/quantum/server.log file instead of their respective log
files. Instead, use log_dir = /var/log/quantum as the default while
still supporting log_file.

Change-Id: Ia6b29de80c8700f4ee27a4e0ba9af241021dc6b9