Update pxe_just_once setting default value to 1

Usually users will set boot order in system to (1) network, (2) disk.
When pxe_just_once option is enabled and DHCP server is managed by Cobbler,
system entry is automatically removed from DHCP server configuration
file. If this option is disabled, Linux installation will restart after
system reboot.

Current default value (0) is error prone and often causes confusion
in beginner Cobbler users. New default value is more intuitive and
friendly.