Use explicit types with get_row

Auto types aren't very neat as return values because they move the burden
of knowledge to the reader. Using an explicit, and somewhat
self-explanatory, type makes it easier to assess the code without knowing
the implementation of the type.

maxscale-system-test/mariadb_func.h

@@ -26,6 +26,8 @@
 #include <string>
 #include <vector>
 
+typedef std::vector<std::string> Row;
+
 /**
  * Opens connection to DB: wropper over mysql_real_connect
  *