Only enable MySQL TRADITIONAL mode if we're running against MySQL

Change I2960435c81d21f5a65ea7fa4dc5c100880cf6aef, which enabled
MySQL TRADITIONAL mode, erroneously would attempt to enable that
MySQL-specific mode on both MySQL and DB2 connections. Only enable
TRADITIONAL mode when actually running against MySQL.

Change-Id: I28c40c47e2515112555f2abdd2ecc4dc5c3399f6
Merge "Only enable MySQL TRADITIONAL mode if we're running against MySQL"