Add log_dir param. by default and allow users to disable it

This commit add log_dir param and configures it to /var/log/keystone
(default location). Set log_dir to false allows users to disable logging
to a directory (usefull if syslog is enabled in order not to log twice).

Change-Id: I83446ff8b70c1cc136c9e1bb24513006ac9cb5a8
Closes-bug: #1282296
Merge "Add log_dir param. by default and allow users to disable it"