ensure log_file is absent when using syslog

When using syslog, we don't need to write in keystone.log file, so
log_file param should be absent to avoid any write in the file.

Example: run keystone-manage db_sync will write in ~/keystone.log
instead of /var/log/syslog.

Closes-bug #1292619
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Change-Id: I9b2613294625f3df27a70f45bd964688a425c88e
(cherry picked from commit 2f4c960a4b6970f3ba4bdf8949ae09f48884b327)