Add option for failover recovery in mysqlmon

The `failover_recovery` option allows failed servers to rejoin the
cluster. This should make using MaxScale with two node clusters easier.

One use case for this is when the replication-manager promotes the last
node in the cluster as the master. When this is done, the slave
configuration is cleared and the read-only mode is disabled. Since the
failover requires that the server is not configured as a slave and that it
is not in read-only mode, it is safe to use `failover_recovery` with
replication-manager.

server/modules/monitor/mysqlmon.h

@@ -77,6 +77,7 @@ typedef struct
     bool failover; /**< If simple failover is enabled */
     int failcount; /**< How many monitoring cycles servers must be
                                    down before failover is initiated */
+    bool failover_recovery; /**< Allow servers to rejoin the cluster in failover mode */
     bool warn_failover; /**< Log a warning when failover happens */
 } MYSQL_MONITOR;