only call get_menu_items once per pxegen.write_all_system_files call

fixes https://github.com/cobbler/cobbler/issues/609

There's no reason to call get_menu_items for each system, so call it once outside the for loop.  There is probably still some optimization in other code related to a full sync, but this seems to be the biggest culprit for the slowdown.

## original
echo -n 'systems: '; cobbler system list | wc -l; time cobbler sync > /dev/null
systems: 142

real	0m12.209s
user	0m0.158s
sys		0m0.031s

## patched
echo -n 'systems: '; cobbler system list | wc -l; time cobbler sync > /dev/null
systems: 142

real	0m4.226s
user	0m0.193s
sys		0m0.019s

echo -n 'systems: '; cobbler system list | wc -l; time cobbler sync > /dev/null
systems: 542

real	0m9.248s
user	0m0.209s
sys	0m0.020s