Add support for Redis Sentinel configuration

Spring Data Redis 1.4.0 introduced Redis Sentinel support. When
specified, RedisConnectionFactory uses the Sentinel configuration to
determine the current master.

Sentinel configuration can be specified using two new properties:
spring.redis.sentinel.master and spring.redis.sentinel.nodes.

For example:

spring.redis.sentinel.master=mymaster # name of redis server
spring.redis.sentinel.nodes=127.0.0.1:26379,127.0.0.1:26380

Alternatively, a bean of type RedisSentinelConfiguration can be declared
and it will be used to configure the connection factory.

Note: At this time, Sentinel support is only available for Jedis

Closes gh-1337