eventscripts: Simplify handling of $service name in "managed" functions

eventscripts: Simplify handling of $service name in "managed" functions

Complicated argument handling was introduced to deal with multiple
services per eventscript.  This was a failure and we split 50.samba.

This simplifies several functions to use global $service_name
unconditionally instead of having an optional argument.

$service_name is no automatically longer set in the functions file.
This means it needs to be explicitly set in 13.per_ip_routing because
this script uses ctdb_service_check_reconfigure().

Eventscript unit test infrastructure needs to set $service_name during
fake service setup, and policy routing tests need to be updated
accordingly.

Signed-off-by: Martin Schwenke <martin@meltin.net>