Fix mxs1509

Temporarily disable the multi-source part of the test. Explicitly set
gtid_slave_pos when changing master. Add missing parameter to
configuration.

maxscale-system-test/cnf/maxscale.cnf.template.mxs1509

@@ -8,6 +8,7 @@ servers=server1,server2
 user=maxskysql
 passwd=skysql
 monitor_interval=1000
+ignore_external_masters=true
 
 [RW Split Router]
 type=service