Making sentinel flush config on +slave

Originally, only the +slave event which occurs when a slave is
reconfigured during sentinelResetMasterAndChangeAddress triggers a flush
of the config to disk.  However, newly discovered slaves don't
apparently trigger this flush but do trigger the +slave event issuance.

So if you start up a sentinel, add a master, then add a slave to the
master (as a way to reproduce it) you'll see the +slave event issued,
but the sentinel config won't be updated with the known-slave entry.

This change makes sentinel do the flush of the config if a new slave is
deteted in sentinelRefreshInstanceInfo.

src/sentinel.c

@@ -1842,6 +1842,7 @@ void sentinelRefreshInstanceInfo(sentinelRedisInstance *ri, const char *info) {
                             atoi(port), ri->quorum, ri)) != NULL)
                 {
                     sentinelEvent(REDIS_NOTICE,"+slave",slave,"%@");
+                    sentinelFlushConfig();
                 }
             }
         }