perf(cartridge/execute_sql): use 'callbre' instead of 'callro' to execute SQL

Basically callbre means that we call replicaset with two additional parameters:
1) prefer_replica = true. We can use read from replicas because transactional consistency is out of Sbroad scope. Basically we do not care about data consistency at all.
2) balance = true. This parameters enables internal vshard load balancing between replicaset nodes in round-robin fashion, with a preference for replicas if prefer_replica=true is also set