Implemented on brian's request.


server-tools/instance-manager/Makefile.am:
  define default config file
server-tools/instance-manager/commands.cc:
  Use specified or default file to edit with SET commands instead of hardcoded file
server-tools/instance-manager/commands.h:
  add member to SET commands
server-tools/instance-manager/instance_map.cc:
  rename first_option -> single_defaults_option, made logging a bit more verbose
server-tools/instance-manager/instance_map.h:
  rename first_option -> single_defaults_file + made it public
server-tools/instance-manager/manager.cc:
  rename first_option -> single_defaults_file
server-tools/instance-manager/mysqlmanager.cc:
  goto generic error label instead of simple return
server-tools/instance-manager/options.cc:
  skip --defaults-extra file and give a message if it was specified, made IM
  to read one config file only
server-tools/instance-manager/options.h:
  added new members to the option structure