upgrade nbench task
Merge branch 'master' of gitlab.baidu.com:xredis/deploy
reconsturct

1. move RedisServer, Nutcracker

[ticket: X]
reconsturct

1. move RedisServer, Nutcracker, Sentinel to a new file lib/server_modules.py
2. cmd with two or more arguments

[ticket: X]
migrate works, must imporove
update README.rst
self code review and make status cmd better.

1. in status, master-slave relation will all come from sentinel, and if
   master down, it will be red
2. backup nutcracker config on reconfig
add sentinel_cmd_reset ..

1. add sentinel_cmd_reset/sentinel_cmd_failover
2. more pre_check for migrate
scheduler works well on migrate
add cleankeys task