Add config parameter for excluding servers from failover

"servers_no_promotion" is a comma-separated list of servers
which cannot be chosen when selecting a new master during failover
(auto or manual), or when automatically selecting a new master
for switchover (currently disabled).

The servers in the list are redirected normally and can be promoted
by switchover when manually selecting a new master.

server/modules/monitor/mysqlmon.h

@@ -73,6 +73,9 @@ typedef struct
     int64_t master_gtid_domain;    /**< Gtid domain currently used by the master */
     bool auto_rejoin;              /**< Attempt to start slave replication on standalone servers or servers
                                         replicating from the wrong master. */
+    int n_excluded;                /**< Number of excluded servers */
+    MXS_MONITORED_SERVER** excluded_servers; /**< Servers banned for master promotion during auto-failover. */
+
     MXS_MONITOR* monitor;
 } MYSQL_MONITOR;