Increase query_retries for mxs1507_migrate_trx

The switchover sometimes fails due to a broken connection when the STOP
SLAVE on the new master is executed. Nothing is logged on the server in
question and the error message simply states that the connection was lost
in the middle of a query.

Increasing the query_retries to 1 reduced the likelihood of failure from
about 1/3 of tests failing to roughly 1/6 of tests failing. Increasing it
to 5 seems to remove it completely. As to what is the real reason this
happens, we do not yet know.

maxscale-system-test/cnf/maxscale.cnf.template.mxs1507_trx_replay

@@ -1,6 +1,8 @@
 [maxscale]
 threads=###threads###
 log_info=1
+query_retries=5
+query_retry_timeout=20
 
 [MySQL Monitor]
 type=monitor