MXS-3410 Rename Mariadb_nodes::flush_hosts() to prepare_for_test()

The function does more than simply executes "FLUSH HOSTS". Also
change return type to bool as that is what it has returned.

system-test/rwsplit_readonly.cpp

@@ -278,7 +278,7 @@ int main(int argc, char* argv[])
     Test->repl->disconnect();
 
     /** Clean up test environment */
-    Test->repl->flush_hosts();
+    Test->repl->prepare_for_test();
     int rval = Test->global_result;
     delete Test;
     return rval;