redis-monitor crashes when the "password" key isn't set in config

[kisabaka]

When a server doesn't need a password but the config for it doesn't have "password" : "some-password", redis-monitor will crash with a KeyError:

Traceback (most recent call last):
  File "./redis-monitor.py", line 300, in <module>
    monitor.run(duration)
  File "./redis-monitor.py", line 252, in run
    if redis_server["password"]:
KeyError: 'password'