MXS-1639 Run user-given sql commands during promotion, demotion and rejoin

The sql queries are given in two text files, defined by options promotion_sql_file
and demotion_sql_file. The files must exist when monitor starts. The files are read
line by line, ignoring empty lines and lines starting with '#'. All other lines
are sent to the server being promoted, demoted or rejoined. Any error in opening
a file, reading it or executing the contents will cause the entire operation to
fail.

The filed defined in demotion_sql_file is also ran when rejoining a server. This
is to ensure a previously failed master is "demoted" properly when it joins the
cluster.

server/modules/monitor/mysqlmon.h

@@ -77,6 +77,8 @@ typedef struct
                                         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. */
+    const char* promote_sql_file;  /**< File with sql commands which are ran to a server being promoted. */
+    const char* demote_sql_file;   /**< File with sql commands which are ran to a server being demoted. */
 
     MXS_MONITOR* monitor;
 } MYSQL_MONITOR;