qan-agent FLUSH SLOW LOGS needlessly creates a GTID event

since FLUSH SLOW LOGS is replicated, it generates a GTID event that executes on the slave and thus contains a GTID event that preclude it from being promoted as a master.  We should change our call to be:

FLUSH NO_WRITE_TO_BINLOG SLOW LOGS

Further this syntax is supported back to 5.5 so we should not have any compatability issues:

https://dev.mysql.com/doc/refman/5.5/en/flush.html

https://dev.mysql.com/doc/refman/5.6/en/flush.html

https://dev.mysql.com/doc/refman/5.7/en/flush.html