Allow caller to disable logging on remote systems

This commit introduces a new possible value for the syslog_facility
configuration option, namely 'off' (or '0', 'no', 'false'…).

If the syslog_facility is set to any of those values, then a constant at
the top of the modules_common code is changed accordingly, effectively
causing the invocation to the logging function to be skipped (as if the
module had set the no_log property of the AnsibleModule base class.

Signed-off-by: martin f. krafft <madduck@madduck.net>