Simplified slapd LogLevel semantics. Refs #2928

- removed rsyslog configuration template
- LogLevel prop value is passed as-is to slapd daemon
- LogLevel default is 0
- rsyslogd is restarted at the end of update event
- log messages are directed to /var/log/slapd
- added logrotate configuration
Removed obsolete nethserver-directory-set-organization.

That action was linked to an obsolete event: nethserver-base-save. No
known UI module invokes it.