Implement setting in mysql_hostgroup_attributes to override global mysql-monitor_slave_lag_when_null

[renecannao]

This is a new feature.

mysql-monitor_slave_lag_when_null is a global variable that applies to all the servers in all hostgroups.
mysql_hostgroup_attributes allows to specify settings that are specific for each hostgroup, thus it allows to override global setting.

Therefore we should be able to override mysql-monitor_slave_lag_when_null in mysql_hostgroup_attributes