Add support to monitor multiple redis masters for one redis sentinel

[freiman]

Redis sentinel have support to monitor multiple redis masters and participate in failover. It would be nice to add support for this. Or at least to implement another issue #223 which can also solved this problem.

It can be implemented like this:

redis::sentinel::masters {'testmaster1':
  redis_host => "...."
}

[basti-nis]

This PR will bring this feature #356