do not catch system exit in ns_server.py
do not catch system exit in ns_server.py

cuihaoliang/zerovm,cuihaoliang/zerovm,zerovm/zerovm,cuihaoliang/zerovm,zerovm/zerovm,zerovm/zerovm,cuihaoliang/zerovm,zerovm/zerovm,cuihaoliang/zerovm,zerovm/zerovm