MXS-1533: Add automatic join feature

When enabled, the monitor will redirect servers to replicate from the
current master. Standalone servers and servers replicating from a slave
are redirected.

server/modules/monitor/mysqlmon.h

@@ -72,6 +72,8 @@ typedef struct
     bool verify_master_failure;    /**< Whether master failure is verified via slaves */
     int master_failure_timeout;    /**< Time in seconds to wait before doing failover */
     int64_t master_gtid_domain;    /**< Gtid domain currently used by the master */
+    bool auto_join_cluster;        /**< Attempt to start slave replication on standalone servers or servers
+                                        replicating from the wrong master. */
     MXS_MONITOR* monitor;
 } MYSQL_MONITOR;