Make use_syslog=True log to syslog via /dev/log

After upgrade from Icehouse to Juno OpenStack services don't log to
syslog via /dev/log anymore. This behaviour was introduced in commit
1188d88a73e75da5c143d0dfad34e10dad0daee0. While /dev/log is arguably
not a very portable default, breaking the backwards compatibility is
even worse.

This commit restores the original behaviour. /dev/log portability
issues will be addressed separately.

Closes-Bug: #1385295
Related-Bug: #1391428

Change-Id: I4e84bca70eee6d6aa8d7525fe38d65ba235dd6b0
Merge "Make use_syslog=True log to syslog via /dev/log"