FEATURE: allow use of redis sentinel via redis_sentinels

Use: DISCOURSE_REDIS_SENTINELS and DISCOURSE_REDIS_HOST to configure redis
sentinel

app/models/global_setting.rb

@@ -31,6 +31,25 @@ class GlobalSetting
     {"production" => hash}
   end
 
+  def self.redis_config
+    @config ||=
+      begin
+        c = {}
+        c[:host] = redis_host if redis_host
+        c[:port] = redis_port if redis_port
+        c[:password] = redis_host if redis_password.present?
+        c[:db] = redis_db if redis_db != 0
+        c[:db] = 1 if Rails.env == "test"
+        if redis_sentinels.present?
+          c[:sentinels] = redis_sentinels.split(",").map do |address|
+            host,port = address.split(":")
+            {host: host, port: port}
+          end.to_a
+        end
+        c.freeze
+      end
+  end
+
 
   class BaseProvider
     def self.coerce(setting)