Add function to read database host from redis

lib/cartodb/carto_data.js

@@ -100,6 +100,26 @@ module.exports = function() {
         });
     };
 
+    /**
+     * Get the database host this particular subdomain/username
+     *
+     * @param req - standard express req object. importantly contains host information
+     * @param callback
+     */
+    me.getDatabaseHost= function(req, callback) {
+        // strip subdomain from header host
+        var username = req.headers.host.split('.')[0];
+        var redisKey = _.template(this.user_key, {username: username});
+
+        this.retrieve(this.user_metadata_db, redisKey, 'database_host', function(err, dbname) {
+          if ( err ) callback(err, null);
+          else if ( dbname === null ) {
+            callback(new Error("missing " + username + "'s dbuser in redis (try CARTODB/script/restore_redis)"), null);
+          }
+          else callback(err, dbname);
+        });
+    };
+
     /**
      * Check the user map key for this particular subdomain/username
      *