MXS-1444: Add monitor parameter alteration

The parameter handling for monitors can now be done in a consistent manner
by establishing a rule that the monitor owns the parameter object as long
as it is running. This will allow parameters to be added and removed
safely both from outside and inside monitors.

Currently this functionality is only used by mysqlmon to disable failover
after an attempt to perform a failover has failed.

server/modules/monitor/mysqlmon/mysql_mon.cc

@@ -1878,6 +1878,8 @@ monitorMain(void *arg)
                 MXS_ALERT("Failed to perform failover, disabling failover functionality. "
                           "To enable failover functionality, manually set 'failover' to "
                           "'true' for monitor '%s' via MaxAdmin or the REST API.", mon->name);
+
+                mon_alter_parameter(handle->monitor, CN_FAILOVER, "false");
                 handle->failover = false;
             }
         }