bgp/operator/ssh: Enable to quit gracefully
bgp/operator/ssh: Enable to quit gracefully

Currently, socket.error will be raised when 'quit' command is typed.
This patch enables to quit SSH session without tracebacks.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

iwaseyusuke/ryu,pichuang/ryu,fkakuma/ryu,TakeshiTseng/ryu,osrg/ryu,osrg/ryu,shinpeimuraoka/ryu,iwaseyusuke/ryu,gareging/SDN_Framework,lagopus/ryu-lagopus-ext,shinpeimuraoka/ryu,osrg/ryu,fkakuma/ryu,TakeshiTseng/ryu,gareging/SDN_Framework,TakeshiTseng/ryu,shinpeimuraoka/ryu,gareging/SDN_Framework,iwaseyusuke/ryu,pichuang/ryu,shinpeimuraoka/ryu,lagopus/ryu-lagopus-ext,lagopus/ryu-lagopus-ext,TakeshiTseng/ryu,gareging/SDN_Framework,shinpeimuraoka/ryu,osrg/ryu,fujita/ryu,fkakuma/ryu,TakeshiTseng/ryu,fkakuma/ryu,iwaseyusuke/ryu,lagopus/ryu-lagopus-ext,gareging/SDN_Framework,fujita/ryu,osrg/ryu,iwaseyusuke/ryu,fkakuma/ryu,fujita/ryu,pichuang/ryu,fujita/ryu,pichuang/ryu,fujita/ryu,pichuang/ryu,lagopus/ryu-lagopus-ext