Support connecting to Redis Sentinel

Needed for Redis HA on gitlab.com gitlab-com/operations#5 (closed).

Means adding sentinel config in config.yml: list of sentinels, master name. And when sentinel is enabled, before connecting to redis, first run redis-cli SENTINEL get-master-addr-by-name mymaster to get the address of the current master.

Since we now have redis-rb library vendorized, we can rely on it to discover / handle sentinel connection itself.

Related to: https://gitlab.com/gitlab-org/gitlab-ce/issues/3355

---

Following gitlab's resque.yml syntax, for config.yml we are going to add a sentinelskey in redis:

# Redis settings used for pushing commit notices to gitlab
redis:
  bin: /usr/bin/redis-cli
  # host: 127.0.0.1
  # port: 6379
  # pass: redispass # Allows you to specify the password for Redis
  database: 0
  socket: /var/run/redis/redis.sock # Comment out this line if you want to use TCP or Sentinels
  namespace: resque:gitlab
  sentinels: 
    -
      host: localhost
      port: 26380 # point to sentinel, not to redis port
    -
      host: slave2
      port: 26381 # point to sentinel, not to redis port