Rename parameter 'syslog_facility' into 'log_facility'

The previous name was a mistake, log_facility is already used by all other
openstack modules. This shouldn't bring compatibility issues because the
parameter was only added a few days ago.

Change-Id: Ifcc2f0daf935610fdbac45ac5badf0a4f904b7d3
Merge "Rename parameter 'syslog_facility' into 'log_facility'"