Support for redis sentinel in redis input and output

Redis supports failover through a separate service called Sentinel (http://redis.io/topics/sentinel). I added support for it to the output and input modules using the redis-sentinel wrapper library (https://github.com/flyerhzm/redis-sentinel).
Somehow I feel more comfortable without autodetection here. Another option would be to have a configuration option like redis_type with options server and sentinel. Explicit type definition would keep the init code simple and less error-prone.

The current host variable is validated as a string, so we'd need to introduce an array variable anyway. Then we should also define what it means to have multiple non-sentinel hosts defined (unless we just get rid of the validation?). On the output side there already is a failover mechanism, and therefore also a configurable array of hosts, so it would be easier to get rid of the sentinel specific host and port definitions there.