Check "auth event notification" param in log_json

Check "auth event notification" param in log_json

When Samba is compiled with JSON support it logs an error message
at log level 3 by default for every authentication event:

get_auth_event_server: Failed to find 'auth_event' registered on the
message bus to send JSON authentication events to:
NT_STATUS_OBJECT_NAME_NOT_FOUND

This patch adjusts the log_json function to consider the smb.conf
parameter "auth event notification" before calling auth_message_send.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13381

Signed-off-by: Arvid Requate <requate@univention.de>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Apr 12 08:12:39 CEST 2018 on sn-devel-144