REDIS conf (changes from the defaults):
daemonize yes
logfile /path/to/log/redis.log
#save 900 1
#save 300 10
#save 60 10000
maxmemory 2g
maxmemory-policy allkeys-lru
appendfsync no

logstash.conf:
output {
  redis {
    host => "centralized-logstash.example.com"
    data_type => [ "list" ]
    key => "logstash"
  }
}

logstash-collector.conf:
input {
  redis {
    host => "centralized-logstash.example.com"
    data_type => [ "list" ]
    key => "logstash"
    type => "all"
  }
}