server-tools/instance-manager/guardian.cc:
  1. Removed unused stop_instances_arg from request_shutdown() and
  stop_instances() methods.
  
  2. Changed log-output statements so that instance name is passed
  correctly (char *, not LEX_STRING)
server-tools/instance-manager/guardian.h:
  Removed unused stop_instances_arg from request_shutdown() and
  stop_instances() methods.
server-tools/instance-manager/instance.cc:
  Removed unused stop_instances_arg from request_shutdown() and
  stop_instances() methods.
server-tools/instance-manager/listener.cc:
  Be more verbose in log.
server-tools/instance-manager/manager.cc:
  Removed unused stop_instances argument.