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

This commit add log_dir param and configures it to /var/log/cinder
(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: I8b8d72f8c9551ffb1a4f73575013d70e985f274c
Closes-bug: #1282296
Merge "Add log_dir param. by default and allow users to disable it"