Galeramon to set the preferred donor nodes

Galeramon should be able to control which nodes are the preferred as the donor with the wsrep_sst_donor variable. This would allow joining nodes to choose "slave" nodes which will allow MaxScale to influence the donor choosing process.

The monitor could execute SELECT @@wsrep_node_name; on a non-master node of the cluster and then assign the preferred SST donor node with SET GLOBAL wsrep_sst_donor=my_donor_node1.

MariaDB [(none)]> select @@wsrep_node_name;

+-----------------------+

| @@wsrep_node_name     |

+-----------------------+

| localhost.localdomain |

+-----------------------+

1 row in set (0.00 sec)

MariaDB [(none)]> set global wsrep_sst_donor = "localhost.localdomain";

Query OK, 0 rows affected (0.00 sec)