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/mxs1506_no_master.cpp

@@ -17,7 +17,7 @@ bool query(TestConnections& test)
     test.maxscales->connect();
     execute_query_silent(test.maxscales->conn_rwsplit[0], "SET @a = 1") == 0;
     sleep(5);
-    auto row = get_row(test.maxscales->conn_rwsplit[0], "SELECT @a");
+    Row row = get_row(test.maxscales->conn_rwsplit[0], "SELECT @a");
     test.maxscales->disconnect();
     return row[0] == "1";
 }